diff --git a/mock/index.js b/mock/index.js
index 15e07afa..7d9609ab 100644
--- a/mock/index.js
+++ b/mock/index.js
@@ -11,7 +11,10 @@ export function mockXHR() {
   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)
   }