diff --git a/transport_test.go b/transport_test.go
index 005b28a..1fb04e2 100644
--- a/transport_test.go
+++ b/transport_test.go
@@ -344,3 +344,25 @@ func TestRefreshTokenWithTrailingSlashBaseURL(t *testing.T) {
 		t.Fatalf("Unexpected RoundTrip response code: %d", res.StatusCode)
 	}
 }
+
+func TestRoundTripperContract(t *testing.T) {
+	tr := &Transport{
+		token: &accessToken{
+			ExpiresAt: time.Now().Add(1*time.Hour),
+			Token: "42",
+		},
+		tr: roundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			if accept := req.Header.Get("Accept"); accept != "42" {
+				t.Errorf("got unexpected Accept request header in parent RoundTripper: %q", accept)
+			}
+			return nil, nil
+		}),
+	}
+	req := http.NewRequest("GET", "http://localhost", nil)
+	req.Header.Set("Accept", "xxx")
+	tr(req)
+	if accept := req.Header.Get("Accept"); accept != "xxx" {
+		t.Errorf("got unexpected Accept request header in caller: %q", accept)
+	}
+}
+