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I am having a strange experience with "luaL_newmetable" and friends.

Having two modules A and B. Module B is called through A. In both modules the entrance code is:
int luaopen_a (lua_State *L) {                        /* resp. luaopen_b */
/* Set new table for shared C-function environment. */
lua_newtable(L); /* removal of these two */
lua_replace(L, LUA_ENVIRONINDEX);       /* made no difference   */
/* Make metatable for filehandle userdata. */
fprintf(stderr,"stacktop at %d before\n",lua_gettop(L));
luaL_newmetatable(L, "a-id");          /* resp. "b-id" */
fprintf(stderr,"stacktop at %d after\n",lua_gettop(L));
/* Add __tostring function. */
lua_pushcfunction(L, iom_tostring);
lua_setfield(L, -2, "__tostring");

1=> I get bus errors when in the B-module I ask for luaL_newmetatable(L, "a-id") in stead of "b-id". 

2=> Calling luaL_newmetatable with a non-existing key "c-id" instead does not raise the error.

3=> I get bus errors when accessing the stack in B ("a-id" in the entrance code) after lines:
  mytype *m = lua_newuserdata(L, sizeof(mytype));
/* Mark userdata as file structure data, remove from the stack. */
luaL_getmetatable(L, "b-id");
lua_setmetatable(L, -2);

I suspect the problem has something to do with accessing the registry for an existing table. But why? The reference manual clearly states (page 61) "If the registry already has the key tname ..... In both cases pushes onto the stack the final value associated with tname in the registry."

I am at my wits end. Any pointers to the culprit will be appreciated. Thanks in advance.
Hans van der Meer