diff --git a/vscode.proposed.d.ts b/vscode.proposed.d.ts
index 71edb59e6d..d2d0913c6f 100644
--- a/vscode.proposed.d.ts
+++ b/vscode.proposed.d.ts
@@ -164,7 +164,7 @@ declare module 'vscode' {
 		readonly uri: Uri;
 		readonly cellKind: CellKind;
 		readonly document: TextDocument;
-		language: string;
+		readonly language: string;
 		outputs: CellOutput[];
 		metadata: NotebookCellMetadata;
 	}