diff --git a/whatwg-fetch/whatwg-fetch-tests.ts b/whatwg-fetch/whatwg-fetch-tests.ts
index 090e114b5d..3f23abe914 100644
--- a/whatwg-fetch/whatwg-fetch-tests.ts
+++ b/whatwg-fetch/whatwg-fetch-tests.ts
@@ -7,7 +7,7 @@ function test_HeadersCopiedFromHeaders() {
 }
 
 function test_HeadersCopiedFromHash() {
-	var source:HeadersMap = {
+	var source: DOMStringMap = {
 		'Content-Type': 'application/json'
 	};
 	return new Headers(source);