diff --git a/src/vs/workbench/api/node/extHostDocumentSaveParticipant.ts b/src/vs/workbench/api/node/extHostDocumentSaveParticipant.ts
index 8a391e3d96d..7bd8bd07c8c 100644
--- a/src/vs/workbench/api/node/extHostDocumentSaveParticipant.ts
+++ b/src/vs/workbench/api/node/extHostDocumentSaveParticipant.ts
@@ -13,6 +13,7 @@ import {TPromise} from 'vs/base/common/winjs.base';
 import {ExtHostDocuments} from 'vs/workbench/api/node/extHostDocuments';
 
 export interface TextDocumentWillSaveEvent {
+	document: vscode.TextDocument;
 	waitUntil(t: Thenable<any>): void;
 }
 
@@ -52,16 +53,23 @@ export class ExtHostDocumentSaveParticipant {
 	}
 
 	private _deliverEventAsync(listener: Function, thisArg: any, document: vscode.TextDocument): TPromise<any> {
+
 		const promises: TPromise<any>[] = [];
-		const event = {
+
+		const event: TextDocumentWillSaveEvent = Object.freeze({
+			document,
 			waitUntil(p: Thenable<any>) {
+				if (Object.isFrozen(promises)) {
+					throw illegalState('waitUntil can not be called async');
+				}
 				promises.push(TPromise.wrap(p));
 			}
-		};
+		});
+
 		try {
 			listener.apply(thisArg, [event]);
 		} finally {
-			event.waitUntil = () => { throw illegalState(); };
+			Object.freeze(promises);
 			return TPromise.join(promises).then(() => void 0, err => void 0 /* ignore */);
 		}
 	}
diff --git a/src/vs/workbench/test/node/api/extHostDocumentSaveParticipant.test.ts b/src/vs/workbench/test/node/api/extHostDocumentSaveParticipant.test.ts
index 4934f66e034..19863f67735 100644
--- a/src/vs/workbench/test/node/api/extHostDocumentSaveParticipant.test.ts
+++ b/src/vs/workbench/test/node/api/extHostDocumentSaveParticipant.test.ts
@@ -61,7 +61,23 @@ suite('ExtHostDocumentSaveParticipant', () => {
 		});
 	});
 
-	test('event delivery in subscribe order', () => {
+	test('event delivery, immutable', () => {
+		const participant = new ExtHostDocumentSaveParticipant(documents);
+
+		let event: TextDocumentWillSaveEvent;
+		let sub = participant.onWillSaveTextDocumentEvent(function (e) {
+			event = e;
+		});
+
+		return participant.$participateInSave(resource).then(() => {
+			sub.dispose();
+
+			assert.ok(event);
+			assert.throws(() => event.document = null);
+		});
+	});
+
+	test('event delivery, in subscriber order', () => {
 		const participant = new ExtHostDocumentSaveParticipant(documents);
 
 		let counter = 0;