Wireshark Lua TCP dissector

I have written several UDP dissectors and they all work fine. I am struggling in creating my 1st TCP dissector for a custom protocol. No matter how I register the protocol wireshark seems to either ignore or override my dissector and use a standard decoder on the packet. The custom protocol port number is 8501 and it is always decoded as cmtp-mgt.

Any suggestions on what I am doing wrong would be appreciated.

local new_proto_tcp = Proto("new_traffic","new TCP Protocol")       
local ft =  new_proto_tcp.fields
ft.source_ip = ProtoField.ipv4 ("new.src_ip",  "Source IP address")
ft.source_port = ProtoField.uint16 ("new.src_port",  "Source Port")
ft.destination_ip = ProtoField.ipv4 ("new.dst_ip",  "Destination IP address")
ft.destination_port = ProtoField.uint16 ("new.dst_port",  "Destination Port")
function new_proto_tcp.dissector(tvbuffer,pinfo,tree)
    local new_tr = tree:add(new_proto_tcp,tvbuffer(),"new Protocol Data")
    Packet_content = 0
    new_tr:add(ft.source_ip, tvbuffer(Packet_content+0, 4))
    new_tr:add(ft.destination_ip, tvbuffer(Packet_content+4, 4))    
end
do
tcp_table = DissectorTable.get("tcp.port")
tcp_table:add(8501,new_proto_tcp)
end