diff --git a/test/middleware-mockserver.js b/test/middleware-mockserver.js
index 9dcf51951..c1fb813e0 100644
--- a/test/middleware-mockserver.js
+++ b/test/middleware-mockserver.js
@@ -79,13 +79,7 @@ const mocks = {
 			headers[ "access-control-allow-origin" ] = "*";
 		}
 
-		if ( resp.set ) {
-			resp.set( headers );
-		} else {
-			for ( const key in headers ) {
-				resp.writeHead( 200, { [ key ]: headers[ key ] } );
-			}
-		}
+		resp.writeHead( 200, headers );
 
 		if ( req.query.callback ) {
 			resp.end( `${ cleanCallback( req.query.callback ) }(${ JSON.stringify( {