If the old _index metamethod does not exist then do not use it. This would

cause "attempt to index a nil value" errors when whatever is being made
"strict" did not have a metatable (the global environment for example).

nselib/strict.lua

@@ -60,7 +60,7 @@ function strict ()
     if type(_index) == "function" then
       local v = _index(t, n); -- hook it
       if v ~= nil then return v end
-    else
+    elseif _index ~= nil then
       local v = _index[n];
       if v ~= nil then return v end
     end