diff --git a/fetch.js b/fetch.js
index a6fd1f69..dfd58cfd 100644
--- a/fetch.js
+++ b/fetch.js
@@ -209,6 +209,17 @@ function Body() {
   this.bodyUsed = false
 
   this._initBody = function(body) {
+    /*
+      fetch-mock wraps the Response object in an ES6 Proxy to
+      provide useful test harness features such as flush. However, on
+      ES5 browsers without fetch or Proxy support pollyfills must be used;
+      the proxy-pollyfill is unable to proxy an attribute unless it exists
+      on the object before the Proxy is created. This change ensures
+      Response.bodyUsed exists on the instance, while maintaining the
+      semantic of setting Request.bodyUsed in the constructor before
+      _initBody is called.
+    */
+    this.bodyUsed = this.bodyUsed
     this._bodyInit = body
     if (!body) {
       this._bodyText = ''