diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index b96bb21019..aa22d5a2fd 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -191,10 +191,11 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 
 	res, err := transport.RoundTrip(outreq)
-	if err != nil {
-		p.logf("http: proxy error: %v", err)
-		rw.WriteHeader(http.StatusBadGateway)
-		return
+	if res == nil {
+		res = &http.Response{
+			StatusCode: http.StatusBadGateway,
+			Body:       http.NoBody,
+		}
 	}
 
 	removeConnectionHeaders(res.Header)
@@ -204,12 +205,16 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 
 	if p.ModifyResponse != nil {
-		if err := p.ModifyResponse(res); err != nil {
+		if err != nil {
 			p.logf("http: proxy error: %v", err)
-			rw.WriteHeader(http.StatusBadGateway)
-			res.Body.Close()
-			return
 		}
+		err = p.ModifyResponse(res)
+	}
+	if err != nil {
+		p.logf("http: proxy error: %v", err)
+		rw.WriteHeader(http.StatusBadGateway)
+		res.Body.Close()
+		return
 	}
 
 	copyHeader(rw.Header(), res.Header)
diff --git a/src/net/http/httputil/reverseproxy_test.go b/src/net/http/httputil/reverseproxy_test.go
index 2232042d3e..822828e5c0 100644
--- a/src/net/http/httputil/reverseproxy_test.go
+++ b/src/net/http/httputil/reverseproxy_test.go
@@ -631,6 +631,35 @@ func TestReverseProxyModifyResponse(t *testing.T) {
 	}
 }
 
+// Issue 21255. Test ModifyResponse when an error from transport.RoundTrip
+// occurs, and that the proxy returns StatusOK.
+func TestReverseProxyModifyResponse_OnError(t *testing.T) {
+	// Always returns an error
+	errBackend := httptest.NewUnstartedServer(nil)
+	errBackend.Config.ErrorLog = log.New(ioutil.Discard, "", 0) // quiet for tests
+	defer errBackend.Close()
+
+	rpURL, _ := url.Parse(errBackend.URL)
+	rproxy := NewSingleHostReverseProxy(rpURL)
+	rproxy.ModifyResponse = func(resp *http.Response) error {
+		// Will be set for a non-nil error
+		resp.StatusCode = http.StatusOK
+		return nil
+	}
+
+	frontend := httptest.NewServer(rproxy)
+	defer frontend.Close()
+
+	resp, err := http.Get(frontend.URL)
+	if err != nil {
+		t.Fatalf("failed to reach proxy: %v", err)
+	}
+	if resp.StatusCode != http.StatusOK {
+		t.Errorf("err != nil: got res.StatusCode %d; expected %d", resp.StatusCode, http.StatusOK)
+	}
+	resp.Body.Close()
+}
+
 // Issue 16659: log errors from short read
 func TestReverseProxy_CopyBuffer(t *testing.T) {
 	backendServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {