diff --git a/cmd/csaf_provider/actions.go b/cmd/csaf_provider/actions.go
index 8d53083..1e4df7d 100644
--- a/cmd/csaf_provider/actions.go
+++ b/cmd/csaf_provider/actions.go
@@ -180,7 +180,8 @@ func (c *controller) upload(r *http.Request) (interface{}, error) {
 	// Extract real TLP from document.
 	if t == tlpCSAF {
 		if t = tlp(strings.ToLower(ex.tlpLabel)); !t.valid() || t == tlpCSAF {
-			return nil, fmt.Errorf("not a valid TL: %s", ex.tlpLabel)
+			return nil, fmt.Errorf(
+				"valid TLP label missing in document (found '%s')", t)
 		}
 	}