diff --git a/userdata.go b/userdata.go
index ecd8e41..59a082b 100644
--- a/userdata.go
+++ b/userdata.go
@@ -27,7 +27,7 @@ func UnmarshalUserData(b []byte) (u *UserData, err error) {
 }
 func MarshalUserData(u *UserData) ([]byte, error) {
 	if u == nil {
-		return
+		return nil, nil
 	}
 	return yaml.Marshal(u)
 }