diff --git a/VERSION b/VERSION
index 1d0ba9e..267577d 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.4.0
+0.4.1
diff --git a/gock/common.go b/gock/common.go
index 0b440fb..d34a6d8 100644
--- a/gock/common.go
+++ b/gock/common.go
@@ -20,10 +20,15 @@ type TransportSetter interface {
 // annonymous function.
 type RoundTripper func(req *http.Request) (*http.Response, error)
 
+// RoundTrip implementation to delegate to an anonymous function.
+func (r RoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	return r(req)
+}
+
 // NewErrorRoundTripper static function to create new net/http.RoundTripper that
 // can be used to test with default http errors while using simple anonymous
 // functions.
-func NewErrorRoundTripper(err error) RoundTripper {
+func NewErrorRoundTripper(err error) http.RoundTripper {
 	return RoundTripper(func(_ *http.Request) (*http.Response, error) {
 		return nil, err
 	})
diff --git a/gock/common_test.go b/gock/common_test.go
index d92ff38..7b747b4 100644
--- a/gock/common_test.go
+++ b/gock/common_test.go
@@ -57,7 +57,7 @@ func TestNewErrorRoundTripper(t *testing.T) {
 			roundTripper := gock.NewErrorRoundTripper(param.err)
 
 			// When
-			response, err := roundTripper(
+			response, err := roundTripper.RoundTrip(
 				&http.Request{Method: http.MethodGet})
 
 			// Then