diff --git a/vscode.proposed.d.ts b/vscode.proposed.d.ts
index a447086312..fde3199ea4 100644
--- a/vscode.proposed.d.ts
+++ b/vscode.proposed.d.ts
@@ -218,6 +218,7 @@ declare module 'vscode' {
 		readonly fileName: string;
 		readonly viewType: string;
 		readonly isDirty: boolean;
+		readonly isUntitled: boolean;
 		readonly cells: NotebookCell[];
 		languages: string[];
 		displayOrder?: GlobPattern[];