The compatibility implementation of lua_setuservalue is just a thin macro over the native implementations of the older Lua versions (lua_setfenv for Lua 5.1, and lua_setuservalue for Lua 5.2). As a consequence, lua_setuservalue does not accept values other than tables (for Lua 5.1), or tables and nil (for Lua 5.2).

See also lua_getuservalue.