additionally return 'error' from 'ToCreateServerMap'
diff --git a/openstack/compute/v2/extensions/diskconfig/requests_test.go b/openstack/compute/v2/extensions/diskconfig/requests_test.go
index 1f4f626..e3c26d4 100644
--- a/openstack/compute/v2/extensions/diskconfig/requests_test.go
+++ b/openstack/compute/v2/extensions/diskconfig/requests_test.go
@@ -29,7 +29,9 @@
 			}
 		}
 	`
-	th.CheckJSONEquals(t, expected, ext.ToServerCreateMap())
+	actual, err := ext.ToServerCreateMap()
+	th.AssertNoErr(t, err)
+	th.CheckJSONEquals(t, expected, actual)
 }
 
 func TestRebuildOpts(t *testing.T) {