diff --git a/extensions/vscode-notebook-tests/src/notebook.test.ts b/extensions/vscode-notebook-tests/src/notebook.test.ts
index 658500995f7..062c55cb1cb 100644
--- a/extensions/vscode-notebook-tests/src/notebook.test.ts
+++ b/extensions/vscode-notebook-tests/src/notebook.test.ts
@@ -329,7 +329,7 @@ suite('metadata', () => {
 		await waitFor(500);
 		assert.equal(vscode.notebook.activeNotebookEditor !== undefined, true, 'notebook first');
 		assert.equal(vscode.notebook.activeNotebookEditor!.document.metadata.custom['testMetadata'] as boolean, false);
-		assert.equal(vscode.notebook.activeNotebookEditor!.selection?.metadata.custom['testCellMetadata'] as boolean, true);
+		assert.equal(vscode.notebook.activeNotebookEditor!.selection?.metadata.custom!['testCellMetadata'] as number, 123);
 		assert.equal(vscode.notebook.activeNotebookEditor!.selection?.language, 'typescript');
 	});
 });
diff --git a/extensions/vscode-notebook-tests/src/notebookTestMain.ts b/extensions/vscode-notebook-tests/src/notebookTestMain.ts
index e7320ffa8bf..6cdd3619de7 100644
--- a/extensions/vscode-notebook-tests/src/notebookTestMain.ts
+++ b/extensions/vscode-notebook-tests/src/notebookTestMain.ts
@@ -29,7 +29,7 @@ export function activate(context: vscode.ExtensionContext): any {
 						cellKind: vscode.CellKind.Code,
 						outputs: [],
 						metadata: {
-							custom: { testCellMetadata: true }
+							custom: { testCellMetadata: 123 }
 						}
 					}
 				]
diff --git a/src/vs/vscode.proposed.d.ts b/src/vs/vscode.proposed.d.ts
index cbd20cb0a48..48556963886 100644
--- a/src/vs/vscode.proposed.d.ts
+++ b/src/vs/vscode.proposed.d.ts
@@ -1594,7 +1594,7 @@ declare module 'vscode' {
 		/**
 		 * Additional attributes of a cell metadata.
 		 */
-		[key: string]: any;
+		custom?: { [key: string]: any };
 	}
 
 	export interface NotebookCell {