[XQuery] BEA_023

XQuery: (potentially) editorial

Section 3.1.5 (Function calls) when it describes the
function conversion rules, it says that in case of
a function invocation from a different module the
type test has to be true in both modules.

This  is surprising. Does this mean that we are allowed
to write in a library module:

define function f($x as ns:shoeSize?)
(: ns:shoeSize derives from xs:integer here :)
{()}

while the importing module contains:

declare external variable $y as ns:shoeSize?;
(: ns:shoeSize derives from String here :)

f($y)

and have the call f($y) succeed  if $y is bound to the
empty sequence ?

This is surprising. Did I misunderstand something?
Is this is not allowed, where is the text that forbids this case?(maybe)