Lua dissector failing: occasionally returns nil values instead of TCP endpoints for valid packets

Hi all. I have a small dissector, which keeps returning nil values for a normal TCP packets from time to time.

For some reason, even if pinfo.net_src is set, ip_src_f() may return nil. Also, I can't get how to decode ip_len_f() return, which is userdata, to number.

    do
        local testdissector = Proto("Test", "Test");
    ip_src_f = Field.new("ip.src")
    ip_dst_f = Field.new("ip.dst")
    tcp_src_f = Field.new("tcp.srcport")
    tcp_dst_f = Field.new("tcp.dstport")
    time_f =  Field.new("frame.time_epoch")
    ip_len_f = Field.new("ip.len")
    frame_len_f = Field.new("frame.len")
    tcp_hlen_f = Field.new("tcp.hdr_len")
    tcp_ack = Field.new("tcp.flags.ack")
    tcp_syn = Field.new("tcp.flags.syn")
    tcp_psh = Field.new("tcp.flags.push")
    tcp_rst = Field.new("tcp.flags.reset")
    tcp_fin = Field.new("tcp.flags.fin")

    function testdissector.dissector(tvbuffer, pinfo, treeitem)
        if 1 == 1 then ---sport then

           local sport = tcp_src_f() -- alright
           local dport = tcp_dst_f() -- alright
           local saddr = pinfo.net_src -- 50% nil
           local daddr = pinfo.net_dst -- 50% nil
       --- do something
           end
     return -- nil, to comply with no-reassembly requirements
     end
register_postdissector(testdissector)

end

TCP reassembly is turned off, HTTP dissector is disabled