Ruby Client prepend and append methods return exception subsequent get

A value appended/prepended with append/prepend does not get appended/prepended. Subsequent get returns error:

ver =client.set("foo", "Hello")
client.prepend("foo", " World",:cas => ver)
client.get("foo")

`get': unable to convert value for key "foo": 399: unexpected token at '"Hello"]' (Couchbase::Error::ValueFormat)