GetElementById "Object Required" Error

I've got what should be a simple assignment of either an element value
or a default string to a variable, but when the element doesn't exist I
get an "Object required" error rather than an assignment of the default
value.

I originally used the simple statement:

var v = document.getElementById('foo').value ||
parent.document.getElementById('foo').value ||
'unknown'
document.writeln(v);

This caused the "Object required" error if there was no element, so I
added some error checking:

var v = (document.getElementById)
? document.getElementById('foo').value
: (parent.document.getElementById)
? parent.document.getElementById('foo').value
: 'unknown'
document.writeln(v);

Unfortunately, this still results in an "Object required" error when
the element is not present in the document or its parent.