diff --git a/src/vs/base/parts/quickinput/browser/quickInput.ts b/src/vs/base/parts/quickinput/browser/quickInput.ts
index 372fe2882c18dd1b0bc72aff5d6f0db5d0db9a3c..3479a6bbead40d4b6af8b4f1ab08960aba37ef0d 100644
--- a/src/vs/base/parts/quickinput/browser/quickInput.ts
+++ b/src/vs/base/parts/quickinput/browser/quickInput.ts
@@ -724,7 +724,7 @@ class QuickPick<T extends IQuickPickItem> extends QuickInput implements IQuickPi
 				this._selectedItems = selectedItems as T[];
 				this.onDidChangeSelectionEmitter.fire(selectedItems as T[]);
 				if (selectedItems.length) {
-					this.onDidAcceptEmitter.fire({ inBackground: (event as MouseEvent).button === 1 /* mouse middle click */ });
+					this.onDidAcceptEmitter.fire({ inBackground: event instanceof MouseEvent && event.button === 1 /* mouse middle click */ });
 				}
 			}));
 			this.visibleDisposables.add(this.ui.list.onChangedCheckedElements(checkedItems => {