diff --git a/src/common/lspLanguageFeatures.ts b/src/common/lspLanguageFeatures.ts
index 00869daa..db9a7425 100644
--- a/src/common/lspLanguageFeatures.ts
+++ b/src/common/lspLanguageFeatures.ts
@@ -153,7 +153,7 @@ function toDiagnostics(resource: Uri, diag: lsTypes.Diagnostic): editor.IMarkerD
 //#region CompletionAdapter
 
 export interface ILanguageWorkerWithCompletions {
-	doComplete(uri: string, position: lsTypes.Position): Promise<lsTypes.CompletionList>;
+	doComplete(uri: string, position: lsTypes.Position): Promise<lsTypes.CompletionList | null>;
 }
 
 export class CompletionAdapter<T extends ILanguageWorkerWithCompletions>
@@ -481,7 +481,7 @@ export class DocumentHighlightAdapter<T extends ILanguageWorkerWithDocumentHighl
 		model: editor.IReadOnlyModel,
 		position: Position,
 		token: CancellationToken
-	): Promise<languages.DocumentHighlight[]> {
+	): Promise<languages.DocumentHighlight[] | undefined> {
 		const resource = model.uri;
 
 		return this._worker(resource)
@@ -501,7 +501,7 @@ export class DocumentHighlightAdapter<T extends ILanguageWorkerWithDocumentHighl
 }
 
 function toDocumentHighlightKind(
-	kind: lsTypes.DocumentHighlightKind
+	kind: lsTypes.DocumentHighlightKind | undefined
 ): languages.DocumentHighlightKind {
 	switch (kind) {
 		case lsTypes.DocumentHighlightKind.Read:
@@ -519,7 +519,7 @@ function toDocumentHighlightKind(
 //#region DefinitionAdapter
 
 export interface ILanguageWorkerWithDefinitions {
-	findDefinition(uri: string, position: lsTypes.Position): Promise<lsTypes.Location>;
+	findDefinition(uri: string, position: lsTypes.Position): Promise<lsTypes.Location | null>;
 }
 
 export class DefinitionAdapter<T extends ILanguageWorkerWithDefinitions>
@@ -531,7 +531,7 @@ export class DefinitionAdapter<T extends ILanguageWorkerWithDefinitions>
 		model: editor.IReadOnlyModel,
 		position: Position,
 		token: CancellationToken
-	): Promise<languages.Definition> {
+	): Promise<languages.Definition | undefined> {
 		const resource = model.uri;
 
 		return this._worker(resource)
@@ -572,7 +572,7 @@ export class ReferenceAdapter<T extends ILanguageWorkerWithReferences>
 		position: Position,
 		context: languages.ReferenceContext,
 		token: CancellationToken
-	): Promise<languages.Location[]> {
+	): Promise<languages.Location[] | undefined> {
 		const resource = model.uri;
 
 		return this._worker(resource)
@@ -597,7 +597,7 @@ export interface ILanguageWorkerWithRename {
 		uri: string,
 		position: lsTypes.Position,
 		newName: string
-	): Promise<lsTypes.WorkspaceEdit>;
+	): Promise<lsTypes.WorkspaceEdit | null>;
 }
 
 export class RenameAdapter<T extends ILanguageWorkerWithRename>
@@ -610,7 +610,7 @@ export class RenameAdapter<T extends ILanguageWorkerWithRename>
 		position: Position,
 		newName: string,
 		token: CancellationToken
-	): Promise<languages.WorkspaceEdit> {
+	): Promise<languages.WorkspaceEdit | undefined> {
 		const resource = model.uri;
 
 		return this._worker(resource)
@@ -623,7 +623,7 @@ export class RenameAdapter<T extends ILanguageWorkerWithRename>
 	}
 }
 
-function toWorkspaceEdit(edit: lsTypes.WorkspaceEdit): languages.WorkspaceEdit {
+function toWorkspaceEdit(edit: lsTypes.WorkspaceEdit | null): languages.WorkspaceEdit | undefined {
 	if (!edit || !edit.changes) {
 		return void 0;
 	}
@@ -743,7 +743,7 @@ export class DocumentLinkAdapter<T extends ILanguageWorkerWithDocumentLinks>
 	public provideLinks(
 		model: editor.IReadOnlyModel,
 		token: CancellationToken
-	): Promise<languages.ILinksList> {
+	): Promise<languages.ILinksList | undefined> {
 		const resource = model.uri;
 
 		return this._worker(resource)
diff --git a/src/css/cssMode.ts b/src/css/cssMode.ts
index 5d15f9d9..c77ae543 100644
--- a/src/css/cssMode.ts
+++ b/src/css/cssMode.ts
@@ -121,6 +121,6 @@ function asDisposable(disposables: IDisposable[]): IDisposable {
 
 function disposeAll(disposables: IDisposable[]) {
 	while (disposables.length) {
-		disposables.pop().dispose();
+		disposables.pop()!.dispose();
 	}
 }
diff --git a/src/css/cssWorker.ts b/src/css/cssWorker.ts
index cd8c02cc..ac50ba76 100644
--- a/src/css/cssWorker.ts
+++ b/src/css/cssWorker.ts
@@ -61,26 +61,44 @@ export class CSSWorker {
 		}
 		return Promise.resolve([]);
 	}
-	async doComplete(uri: string, position: cssService.Position): Promise<cssService.CompletionList> {
+	async doComplete(
+		uri: string,
+		position: cssService.Position
+	): Promise<cssService.CompletionList | null> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return null;
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let completions = this._languageService.doComplete(document, position, stylesheet);
 		return Promise.resolve(completions);
 	}
-	async doHover(uri: string, position: cssService.Position): Promise<cssService.Hover> {
+	async doHover(uri: string, position: cssService.Position): Promise<cssService.Hover | null> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return null;
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let hover = this._languageService.doHover(document, position, stylesheet);
 		return Promise.resolve(hover);
 	}
-	async findDefinition(uri: string, position: cssService.Position): Promise<cssService.Location> {
+	async findDefinition(
+		uri: string,
+		position: cssService.Position
+	): Promise<cssService.Location | null> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return null;
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let definition = this._languageService.findDefinition(document, position, stylesheet);
 		return Promise.resolve(definition);
 	}
 	async findReferences(uri: string, position: cssService.Position): Promise<cssService.Location[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let references = this._languageService.findReferences(document, position, stylesheet);
 		return Promise.resolve(references);
@@ -90,12 +108,18 @@ export class CSSWorker {
 		position: cssService.Position
 	): Promise<cssService.DocumentHighlight[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let highlights = this._languageService.findDocumentHighlights(document, position, stylesheet);
 		return Promise.resolve(highlights);
 	}
 	async findDocumentSymbols(uri: string): Promise<cssService.SymbolInformation[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let symbols = this._languageService.findDocumentSymbols(document, stylesheet);
 		return Promise.resolve(symbols);
@@ -106,12 +130,18 @@ export class CSSWorker {
 		context: cssService.CodeActionContext
 	): Promise<cssService.Command[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let actions = this._languageService.doCodeActions(document, range, context, stylesheet);
 		return Promise.resolve(actions);
 	}
 	async findDocumentColors(uri: string): Promise<cssService.ColorInformation[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let colorSymbols = this._languageService.findDocumentColors(document, stylesheet);
 		return Promise.resolve(colorSymbols);
@@ -122,6 +152,9 @@ export class CSSWorker {
 		range: cssService.Range
 	): Promise<cssService.ColorPresentation[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let colorPresentations = this._languageService.getColorPresentations(
 			document,
@@ -136,6 +169,9 @@ export class CSSWorker {
 		context?: { rangeLimit?: number }
 	): Promise<cssService.FoldingRange[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let ranges = this._languageService.getFoldingRanges(document, context);
 		return Promise.resolve(ranges);
 	}
@@ -144,6 +180,9 @@ export class CSSWorker {
 		positions: cssService.Position[]
 	): Promise<cssService.SelectionRange[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let ranges = this._languageService.getSelectionRanges(document, positions, stylesheet);
 		return Promise.resolve(ranges);
@@ -152,13 +191,16 @@ export class CSSWorker {
 		uri: string,
 		position: cssService.Position,
 		newName: string
-	): Promise<cssService.WorkspaceEdit> {
+	): Promise<cssService.WorkspaceEdit | null> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return null;
+		}
 		let stylesheet = this._languageService.parseStylesheet(document);
 		let renames = this._languageService.doRename(document, position, newName, stylesheet);
 		return Promise.resolve(renames);
 	}
-	private _getTextDocument(uri: string): cssService.TextDocument {
+	private _getTextDocument(uri: string): cssService.TextDocument | null {
 		let models = this._ctx.getMirrorModels();
 		for (let model of models) {
 			if (model.uri.toString() === uri) {
diff --git a/src/css/monaco.contribution.ts b/src/css/monaco.contribution.ts
index 82fb127d..5b01210a 100644
--- a/src/css/monaco.contribution.ts
+++ b/src/css/monaco.contribution.ts
@@ -113,8 +113,8 @@ export type DiagnosticsOptions = Options;
 
 class LanguageServiceDefaultsImpl implements LanguageServiceDefaults {
 	private _onDidChange = new Emitter<LanguageServiceDefaults>();
-	private _options: Options;
-	private _modeConfiguration: ModeConfiguration;
+	private _options!: Options;
+	private _modeConfiguration!: ModeConfiguration;
 	private _languageId: string;
 
 	constructor(languageId: string, options: Options, modeConfiguration: ModeConfiguration) {
diff --git a/src/css/workerManager.ts b/src/css/workerManager.ts
index 4485c638..d00ac2df 100644
--- a/src/css/workerManager.ts
+++ b/src/css/workerManager.ts
@@ -15,12 +15,13 @@ export class WorkerManager {
 	private _lastUsedTime: number;
 	private _configChangeListener: IDisposable;
 
-	private _worker: editor.MonacoWebWorker<CSSWorker>;
-	private _client: Promise<CSSWorker>;
+	private _worker: editor.MonacoWebWorker<CSSWorker> | null;
+	private _client: Promise<CSSWorker> | null;
 
 	constructor(defaults: LanguageServiceDefaults) {
 		this._defaults = defaults;
 		this._worker = null;
+		this._client = null;
 		this._idleCheckInterval = window.setInterval(() => this._checkIfIdle(), 30 * 1000);
 		this._lastUsedTime = 0;
 		this._configChangeListener = this._defaults.onDidChange(() => this._stopWorker());
@@ -80,7 +81,9 @@ export class WorkerManager {
 				_client = client;
 			})
 			.then((_) => {
-				return this._worker.withSyncedResources(resources);
+				if (this._worker) {
+					return this._worker.withSyncedResources(resources);
+				}
 			})
 			.then((_) => _client);
 	}
diff --git a/src/html/htmlMode.ts b/src/html/htmlMode.ts
index 779c5edf..e974ecc5 100644
--- a/src/html/htmlMode.ts
+++ b/src/html/htmlMode.ts
@@ -163,6 +163,6 @@ function asDisposable(disposables: IDisposable[]): IDisposable {
 
 function disposeAll(disposables: IDisposable[]) {
 	while (disposables.length) {
-		disposables.pop().dispose();
+		disposables.pop()!.dispose();
 	}
 }
diff --git a/src/html/htmlWorker.ts b/src/html/htmlWorker.ts
index 71bc937e..b0930be0 100644
--- a/src/html/htmlWorker.ts
+++ b/src/html/htmlWorker.ts
@@ -37,8 +37,11 @@ export class HTMLWorker {
 	async doComplete(
 		uri: string,
 		position: htmlService.Position
-	): Promise<htmlService.CompletionList> {
+	): Promise<htmlService.CompletionList | null> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return null;
+		}
 		let htmlDocument = this._languageService.parseHTMLDocument(document);
 		return Promise.resolve(
 			this._languageService.doComplete(
@@ -55,12 +58,18 @@ export class HTMLWorker {
 		options: htmlService.FormattingOptions
 	): Promise<htmlService.TextEdit[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let formattingOptions = { ...this._languageSettings.format, ...options };
 		let textEdits = this._languageService.format(document, range, formattingOptions);
 		return Promise.resolve(textEdits);
 	}
-	async doHover(uri: string, position: htmlService.Position): Promise<htmlService.Hover> {
+	async doHover(uri: string, position: htmlService.Position): Promise<htmlService.Hover | null> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return null;
+		}
 		let htmlDocument = this._languageService.parseHTMLDocument(document);
 		let hover = this._languageService.doHover(document, position, htmlDocument);
 		return Promise.resolve(hover);
@@ -70,17 +79,26 @@ export class HTMLWorker {
 		position: htmlService.Position
 	): Promise<htmlService.DocumentHighlight[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let htmlDocument = this._languageService.parseHTMLDocument(document);
 		let highlights = this._languageService.findDocumentHighlights(document, position, htmlDocument);
 		return Promise.resolve(highlights);
 	}
 	async findDocumentLinks(uri: string): Promise<htmlService.DocumentLink[]> {
 		let document = this._getTextDocument(uri);
-		let links = this._languageService.findDocumentLinks(document, null);
+		if (!document) {
+			return [];
+		}
+		let links = this._languageService.findDocumentLinks(document, null! /*TODO@aeschli*/);
 		return Promise.resolve(links);
 	}
 	async findDocumentSymbols(uri: string): Promise<htmlService.SymbolInformation[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let htmlDocument = this._languageService.parseHTMLDocument(document);
 		let symbols = this._languageService.findDocumentSymbols(document, htmlDocument);
 		return Promise.resolve(symbols);
@@ -90,6 +108,9 @@ export class HTMLWorker {
 		context?: { rangeLimit?: number }
 	): Promise<htmlService.FoldingRange[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let ranges = this._languageService.getFoldingRanges(document, context);
 		return Promise.resolve(ranges);
 	}
@@ -98,6 +119,9 @@ export class HTMLWorker {
 		positions: htmlService.Position[]
 	): Promise<htmlService.SelectionRange[]> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return [];
+		}
 		let ranges = this._languageService.getSelectionRanges(document, positions);
 		return Promise.resolve(ranges);
 	}
@@ -105,13 +129,16 @@ export class HTMLWorker {
 		uri: string,
 		position: htmlService.Position,
 		newName: string
-	): Promise<htmlService.WorkspaceEdit> {
+	): Promise<htmlService.WorkspaceEdit | null> {
 		let document = this._getTextDocument(uri);
+		if (!document) {
+			return null;
+		}
 		let htmlDocument = this._languageService.parseHTMLDocument(document);
 		let renames = this._languageService.doRename(document, position, newName, htmlDocument);
 		return Promise.resolve(renames);
 	}
-	private _getTextDocument(uri: string): htmlService.TextDocument {
+	private _getTextDocument(uri: string): htmlService.TextDocument | null {
 		let models = this._ctx.getMirrorModels();
 		for (let model of models) {
 			if (model.uri.toString() === uri) {
diff --git a/src/html/monaco.contribution.ts b/src/html/monaco.contribution.ts
index 30a24b2a..57412060 100644
--- a/src/html/monaco.contribution.ts
+++ b/src/html/monaco.contribution.ts
@@ -14,7 +14,7 @@ export interface HTMLFormatConfiguration {
 	readonly contentUnformatted: string;
 	readonly indentInnerHtml: boolean;
 	readonly preserveNewLines: boolean;
-	readonly maxPreserveNewLines: number;
+	readonly maxPreserveNewLines: number | undefined;
 	readonly indentHandlebars: boolean;
 	readonly endWithNewline: boolean;
 	readonly extraLiners: string;
@@ -114,8 +114,8 @@ export interface LanguageServiceDefaults {
 
 class LanguageServiceDefaultsImpl implements LanguageServiceDefaults {
 	private _onDidChange = new Emitter<LanguageServiceDefaults>();
-	private _options: Options;
-	private _modeConfiguration: ModeConfiguration;
+	private _options!: Options;
+	private _modeConfiguration!: ModeConfiguration;
 	private _languageId: string;
 
 	constructor(languageId: string, options: Options, modeConfiguration: ModeConfiguration) {
@@ -160,7 +160,7 @@ const formatDefaults: Required<HTMLFormatConfiguration> = {
 	contentUnformatted: 'pre',
 	indentInnerHtml: false,
 	preserveNewLines: true,
-	maxPreserveNewLines: null,
+	maxPreserveNewLines: undefined,
 	indentHandlebars: false,
 	endWithNewline: false,
 	extraLiners: 'head, body, /html',
diff --git a/src/html/workerManager.ts b/src/html/workerManager.ts
index 74094e78..e3741bf0 100644
--- a/src/html/workerManager.ts
+++ b/src/html/workerManager.ts
@@ -15,12 +15,13 @@ export class WorkerManager {
 	private _lastUsedTime: number;
 	private _configChangeListener: IDisposable;
 
-	private _worker: editor.MonacoWebWorker<HTMLWorker>;
-	private _client: Promise<HTMLWorker>;
+	private _worker: editor.MonacoWebWorker<HTMLWorker> | null;
+	private _client: Promise<HTMLWorker> | null;
 
 	constructor(defaults: LanguageServiceDefaults) {
 		this._defaults = defaults;
 		this._worker = null;
+		this._client = null;
 		this._idleCheckInterval = window.setInterval(() => this._checkIfIdle(), 30 * 1000);
 		this._lastUsedTime = 0;
 		this._configChangeListener = this._defaults.onDidChange(() => this._stopWorker());
diff --git a/src/tsconfig.json b/src/tsconfig.json
index 707f714d..7d1e23db 100644
--- a/src/tsconfig.json
+++ b/src/tsconfig.json
@@ -1,10 +1,11 @@
 {
 	"compilerOptions": {
+		"declaration": true,
+		"lib": ["dom", "es5", "es2015.collection", "es2015.promise", "es2015.iterable"],
 		"module": "amd",
 		"moduleResolution": "node",
 		"outDir": "../out/amd",
-		"declaration": true,
-		"target": "es5",
-		"lib": ["dom", "es5", "es2015.collection", "es2015.promise", "es2015.iterable"]
+		"strict": true,
+		"target": "es5"
 	}
 }
diff --git a/website/playground/monaco.d.ts.txt b/website/playground/monaco.d.ts.txt
index bfec7b8a..02f42a34 100644
--- a/website/playground/monaco.d.ts.txt
+++ b/website/playground/monaco.d.ts.txt
@@ -7594,7 +7594,7 @@ declare namespace monaco.languages.html {
         readonly contentUnformatted: string;
         readonly indentInnerHtml: boolean;
         readonly preserveNewLines: boolean;
-        readonly maxPreserveNewLines: number;
+        readonly maxPreserveNewLines: number | undefined;
         readonly indentHandlebars: boolean;
         readonly endWithNewline: boolean;
         readonly extraLiners: string;
diff --git a/website/typedoc/monaco.d.ts b/website/typedoc/monaco.d.ts
index bfec7b8a..02f42a34 100644
--- a/website/typedoc/monaco.d.ts
+++ b/website/typedoc/monaco.d.ts
@@ -7594,7 +7594,7 @@ declare namespace monaco.languages.html {
         readonly contentUnformatted: string;
         readonly indentInnerHtml: boolean;
         readonly preserveNewLines: boolean;
-        readonly maxPreserveNewLines: number;
+        readonly maxPreserveNewLines: number | undefined;
         readonly indentHandlebars: boolean;
         readonly endWithNewline: boolean;
         readonly extraLiners: string;