How can I use the PTP(IEEE1588) dissector as a postdissector in my LUA script?

Hi all, I want to decode a PTP(IEEE1588) message stream which is NOT transported via Ethernet. Basic idea was to unpack the stream from my transport layer and then take the PTP dissector used in Wireshark as a post dissector in my LUA script. However attempts to get the dissector handle using "local default_dissector = Dissector.get("ptp")" returned a Lua Error: "bad argument #1 to 'get' (Dissector_get: No such dissector)". Any comments on how the PTP dissector can be accessed? Thanks