Append with :format => :plain seems to corrupt value

>> require 'couchbase'
=> true
>> cb= Couchbase.new
=> #<Couchbase::Bucket:0x10dd89a10 "http://localhost:8091/pools/default/buckets/default/" default_format=:document, default_flags=0x0, quiet=true, connected=true, timeout=2500000>
>> cb.set('karen', 'old')
=> 1585267068834414592
>> cb.append('karen', 'new', :format => :plain)
=> 1657324662872342528
>> cb.get('karen')
Couchbase::Error::ValueFormat: unable to convert value
from /Library/Ruby/Gems/1.8/gems/yajl-ruby-1.1.0/lib/yajl/json_gem/parsing.rb:23:in `get'
from (irb):5
from :0
>>

When I telnet I get this:

Karens-MacBook-Pro:~ kzeller$ telnet localhost 11211
Trying ::1...
Connected to localhost.
Escape character is '^]'.
get karen
VALUE karen 0 8
"old"new

so the value is appended, but somehow the SDK thinks the content is JSON, not plain.