no calling environment

["Brian Weed" <bw@...>]

After recently upgrading to Lua v5.1.1,  I am now getting an error when 
trying to call luaopen_io  (previously known as  lua_iolibopen):

luaopen_io->lua_replace->lua_G_runerror

lapi.c line 203: 

/* explicit test for incompatible code */
if (...)
   luaG_runerror(L, "no calling environment")

Can someone tell me why one would get this error?  I'm not messing with 
environments (not on purpose, anyway).

Brian