diff --git a/src/vs/vscode.d.ts b/src/vs/vscode.d.ts
index 35a6a58cc03..296649cd8f9 100644
--- a/src/vs/vscode.d.ts
+++ b/src/vs/vscode.d.ts
@@ -201,7 +201,9 @@ declare module 'vscode' {
 		 * Get a word-range at the given position. By default words are defined by
 		 * common separators, like space, -, _, etc. In addition, per languge custom
 		 * [word definitions](#LanguageConfiguration.wordPattern) can be defined. It
-		 * is also possible to provide a custom regular expression.
+		 * is also possible to provide a custom regular expression. *Note* that a
+		 * custom regular expression must not match the empty string and that it will
+		 * be ignored if it does.
 		 *
 		 * The position will be [adjusted](#TextDocument.validatePosition).
 		 *
diff --git a/src/vs/workbench/api/node/extHostDocumentData.ts b/src/vs/workbench/api/node/extHostDocumentData.ts
index 96cb2aec92d..e52493a31e0 100644
--- a/src/vs/workbench/api/node/extHostDocumentData.ts
+++ b/src/vs/workbench/api/node/extHostDocumentData.ts
@@ -243,6 +243,7 @@ export class ExtHostDocumentData extends MirrorModel {
 	private _getWordRangeAtPosition(_position: vscode.Position, regexp?: RegExp): vscode.Range {
 		let position = this._validatePosition(_position);
 		if (!regexp || regExpLeadsToEndlessLoop(regexp)) {
+			console.warn(`[getWordRangeAtPosition]: ignoring custom regexp '${regexp.source}' because it matches the empty string.`);
 			regexp = getWordDefinitionFor(this._languageId);
 		}
 		let wordAtText = getWordAtText(