Why does undef work fine the second time in requirejs?

Question

RequireJS

There is an added custom module described through

define(['custom'], function(){ return { load: function(name, req, onLoad, config){
onLoad.fromText('smth js code');
} }; });

So the problem is that it
require.undef('custom!smth_module');
works fine only the second time, i.e. require->undef->require doesn't work. The second require raises an error because custom module is not hooked and smth_module tries to load from the standard namespace instead of the logic described in it.
But at the same time, if you do require->undef->require->undef->require, then the third require works as it should - without errors, and correctly calls custom logic.
What the hell, who knows?