diff --git a/src/vs/vscode.proposed.d.ts b/src/vs/vscode.proposed.d.ts
index 506f82a9d8d..b5dabbf6b3a 100644
--- a/src/vs/vscode.proposed.d.ts
+++ b/src/vs/vscode.proposed.d.ts
@@ -765,11 +765,11 @@ declare module 'vscode' {
 	export namespace debug {
 
 		/**
-		 * Stop the given debug session or stop all debug sessions if no session is specified.
-		 * @param session The [debug session](#DebugSession) to stop or `undefined` for stopping all sessions.
+		 * Stop the given debug session or stop all debug sessions if session is omitted.
+		 * @param session The [debug session](#DebugSession) to stop; if omitted all sessions are stopped.
 		 * @return A thenable that resolves when the sessions could be stopped successfully.
 		 */
-		export function stopDebugging(session: DebugSession | undefined): Thenable<void>;
+		export function stopDebugging(session?: DebugSession): Thenable<void>;
 	}
 
 	//#endregion
diff --git a/src/vs/workbench/api/common/extHost.api.impl.ts b/src/vs/workbench/api/common/extHost.api.impl.ts
index 68dccbedc5b..97793666ad8 100644
--- a/src/vs/workbench/api/common/extHost.api.impl.ts
+++ b/src/vs/workbench/api/common/extHost.api.impl.ts
@@ -870,7 +870,7 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 				}
 				return extHostDebugService.startDebugging(folder, nameOrConfig, parentSessionOrOptions || {});
 			},
-			stopDebugging(session: vscode.DebugSession | undefined) {
+			stopDebugging(session?: vscode.DebugSession) {
 				return extHostDebugService.stopDebugging(session);
 			},
 			addBreakpoints(breakpoints: vscode.Breakpoint[]) {
diff --git a/src/vs/workbench/api/common/extHostDebugService.ts b/src/vs/workbench/api/common/extHostDebugService.ts
index f9dcbecccef..82121bcb2c6 100644
--- a/src/vs/workbench/api/common/extHostDebugService.ts
+++ b/src/vs/workbench/api/common/extHostDebugService.ts
@@ -51,7 +51,7 @@ export interface IExtHostDebugService extends ExtHostDebugServiceShape {
 	addBreakpoints(breakpoints0: vscode.Breakpoint[]): Promise<void>;
 	removeBreakpoints(breakpoints0: vscode.Breakpoint[]): Promise<void>;
 	startDebugging(folder: vscode.WorkspaceFolder | undefined, nameOrConfig: string | vscode.DebugConfiguration, options: vscode.DebugSessionOptions): Promise<boolean>;
-	stopDebugging(session: vscode.DebugSession | undefined): Promise<void>;
+	stopDebugging(session?: vscode.DebugSession): Promise<void>;
 	registerDebugConfigurationProvider(type: string, provider: vscode.DebugConfigurationProvider, trigger: vscode.DebugConfigurationProviderTriggerKind): vscode.Disposable;
 	registerDebugAdapterDescriptorFactory(extension: IExtensionDescription, type: string, factory: vscode.DebugAdapterDescriptorFactory): vscode.Disposable;
 	registerDebugAdapterTrackerFactory(type: string, factory: vscode.DebugAdapterTrackerFactory): vscode.Disposable;
@@ -302,7 +302,7 @@ export abstract class ExtHostDebugServiceBase implements IExtHostDebugService, E
 		});
 	}
 
-	public stopDebugging(session: vscode.DebugSession | undefined): Promise<void> {
+	public stopDebugging(session?: vscode.DebugSession): Promise<void> {
 		return this._debugServiceProxy.$stopDebugging(session ? session.id : undefined);
 	}