diff --git a/src/mock/index.js b/src/mock/index.js
index a6ab5f14..c21d4e4b 100644
--- a/src/mock/index.js
+++ b/src/mock/index.js
@@ -11,7 +11,9 @@ Mock.XHR.prototype.proxy_send = Mock.XHR.prototype.send
 Mock.XHR.prototype.send = function() {
   if (this.custom.xhr) {
     this.custom.xhr.withCredentials = this.withCredentials || false
-    this.custom.xhr.responseType = this.responseType
+    if (this.responseType) {
+      this.custom.xhr.responseType = this.responseType
+    }
   }
   this.proxy_send(...arguments)
 }