undeclared variable in built-in module if called through invoke

[err:XPDY0002]: "r:foo": undeclared variable is raised for the following scenario.

----------------------
A built-in module such as random declares a variable as follows:

declare variable $r:foo := 1;

----------------------
A library module foo declares a variable and accesses $r:foo in a function.

module namespace foo = "http://foo.com/";

import module namespace r = 'http://www.zorba-xquery.com/modules/random';

declare variable $foo:blub := 1;

declare function foo:bar()
{
  $r:foo
};

----------------------
A main module imports the library module foo and invokes the function foo:bar()

import module namespace invoke = 'http://www.zorba-xquery.com/modules/reflection';
import module namespace foo = "http://foo.com/" at "/home/mbrantner/zorba/build/foo.xq";

invoke:invoke-s ( fn:QName ( 'http://foo.com/', 'bar' ) )