diff --git a/src/vs/workbench/api/common/extHostExtensionService.ts b/src/vs/workbench/api/common/extHostExtensionService.ts
index 328b9327207e4f2068bfab6cf374c622d8c5fc69..2f3857ac7f3de18294335f726d6b3723c25565d5 100644
--- a/src/vs/workbench/api/common/extHostExtensionService.ts
+++ b/src/vs/workbench/api/common/extHostExtensionService.ts
@@ -35,6 +35,8 @@ import { IExtHostTunnelService } from 'vs/workbench/api/common/extHostTunnelServ
 import { IExtHostTerminalService } from 'vs/workbench/api/common/extHostTerminalService';
 import { Emitter, Event } from 'vs/base/common/event';
 import { IExtensionActivationHost, checkActivateWorkspaceContainsExtension } from 'vs/workbench/api/common/shared/workspaceContains';
+import { isEqualOrParent } from 'vs/base/common/extpath';
+import { isLinux } from 'vs/base/common/platform';
 
 interface ITestRunner {
 	/** Old test runner API, as exported from `vscode/lib/testrunner` */
@@ -537,6 +539,20 @@ export abstract class AbstractExtHostExtensionService extends Disposable impleme
 
 		const extensionTestsPath = originalFSPath(extensionTestsLocationURI);
 
+		let knowsExtension = false;
+		for (const extension of this._registry.getAllExtensionDescriptions()) {
+			if (extension.extensionLocation.scheme === Schemas.file) {
+				const extensionPath = originalFSPath(extension.extensionLocation);
+				if (isEqualOrParent(extensionTestsPath, extensionPath, !isLinux)) {
+					knowsExtension = true;
+				}
+			}
+		}
+
+		if (!knowsExtension) {
+			return Promise.resolve(undefined);
+		}
+
 		// Require the test runner via node require from the provided path
 		let testRunner: ITestRunner | INewTestRunner | undefined;
 		let requireError: Error | undefined;