strange code in array_in

In array_in(), I see the following code:
 my_extra->element_type = ~element_type;

It seems like it was explicitly changed from InvalidOid to
~element_type. At first I thought it was a mistake, but then I thought
maybe it was to ensure that the next branch was taken even if
element_type == InvalidOid. But the rest of the lookups will surely fail
on InvalidOid, and it seems like it would be easy to test for InvalidOid
at the beginning if we wanted to fail.

Can someone please explain, and perhaps include a comment indicating
what's going on? Or is it just too early and I missed something simple?

Regards,Jeff Davis