frame.len not working as expected in Lua

edit

I want to use the frame.len to further dissect the packet. I used the following logic and it failed:

   frame_len_f     = Field.new("frame.len")
   local frame_len = frame_len_f()
   if frame_len == 62 then 
        local data_data   = data_data_f()
        local s_data_data = tostring(data_data)
        subtree:add(data_data_F,s_data_data)
   end

However, if I convert frame.len to string and compare to "62", it works:

   frame_len_f     = Field.new("frame.len")
   local frame_len = frame_len_f()
   local s_frame_len = tostring(frame_len)
   if s_frame_len == "62" then 
        local data_data   = data_data_f()
        local s_data_data = tostring(data_data)
        subtree:add(data_data_F,s_data_data)
   end

Can anyone explain what I did wrong in the first case. Thanks