diff --git a/src/vs/workbench/api/node/extHostWorkspace.ts b/src/vs/workbench/api/node/extHostWorkspace.ts
index 61b0e73992f..88d2d8d3f25 100644
--- a/src/vs/workbench/api/node/extHostWorkspace.ts
+++ b/src/vs/workbench/api/node/extHostWorkspace.ts
@@ -8,7 +8,7 @@ import URI from 'vs/base/common/uri';
 import { Event, Emitter } from 'vs/base/common/event';
 import { normalize } from 'vs/base/common/paths';
 import { delta as arrayDelta } from 'vs/base/common/arrays';
-import { relative, dirname } from 'path';
+import { relative, posix } from 'path';
 import { Workspace, WorkspaceFolder } from 'vs/platform/workspace/common/workspace';
 import { IWorkspaceData, ExtHostWorkspaceShape, MainContext, MainThreadWorkspaceShape, IMainContext, MainThreadMessageServiceShape } from './extHost.protocol';
 import * as vscode from 'vscode';
@@ -121,7 +121,7 @@ class ExtHostWorkspaceImpl extends Workspace {
 	getWorkspaceFolder(uri: URI, resolveParent?: boolean): vscode.WorkspaceFolder {
 		if (resolveParent && this._structure.get(uri.toString())) {
 			// `uri` is a workspace folder so we check for its parent
-			uri = uri.with({ path: dirname(uri.path) });
+			uri = uri.with({ path: posix.dirname(uri.path) });
 		}
 		return this._structure.findSubstr(uri.toString());
 	}