bucket Update delete the record

I am using the Update function of go-couchbase

        //defining the function first and passing it as an argument

	myfunc := func(current []byte)(updated []byte, err error) {return updated, err }
	
	myb.Update("key123", 1, myfunc)

However, when I run the Update function of the bucket. I checked the couch database. The document with the key of "key123" was disappeared. It seems the Update does not update the value but delete it. What happened? Am I doing something not correct?

Thanks