L.DoFile does not work when i require a so in lua script

[littleningmeng]

I have a lua script a.lua, and in this file I required a Lua so module:

pb = require "pb"
print(pb)

and the pb module is a lua so in the same dir with a.lua
but, when the Golang code below not work

L := lua.NewState()
defer L.Close()
if err := L.DoFile("a.lua"); err != nil {
  panic(err)
}

error message:

panic: <string>:1: module pb not found:
	no field package.preload['pb']
	stat ./pb.lua: no such file or directory
	stat /usr/local/share/lua/5.1/pb.lua: no such file or directory
	stat /usr/local/share/lua/5.1/pb/init.lua: no such file or directory,