calling a dissector fails with tvb subset in lua

Hi,

I have an disssector that calls another post-dissector.

That works fine in Wireshark 2.2.0 with

function my_proto.dissector(tvb,pinfo,tree)
    local ptp_dissector_table = DissectorTable.get("ptp.data")
    local packet_dissector = ptp_dissector_table:get_dissector('ptp') 
    packet_dissector(tvb, pinfo, tree) 
 end

but fails as soon as I use

  packet_dissector(tvb(some_offset), pinfo, tree)

with 'Bad argument #2 to packet_dissector (Tvb expected, got userdata)

Any hints?