diff --git a/src/vs/base/common/network.ts b/src/vs/base/common/network.ts
index 7444e2c3a5f..109504468f8 100644
--- a/src/vs/base/common/network.ts
+++ b/src/vs/base/common/network.ts
@@ -88,7 +88,7 @@ export function xhr(options: IXHROptions): TPromise<XMLHttpRequest> {
 			options.user,
 			options.password
 		);
-		req.responseType = options.responseType || '';
+		req.responseType = (options.responseType as XMLHttpRequestResponseType) || '';
 
 		Object.keys(options.headers || {}).forEach((k) => {
 			req.setRequestHeader(k, options.headers[k]);