[FX.php List] Using global field in a calc

[Roger Price]

This should be really simple!

gGlobal_Field (text)
text_field
cCalc_field (text) - 'if (isempty(text_field);gGlobal_field;text_field))' 
(unstored)

The above resolves fine in FileMaker but when I try to get the value of 
'cCalc_field' from a found record via FX.php it displays correctly when 
'text_field' is empty but displays some other value (not now in the database 
as far as I can tell) when it isn't!

Any ideas why?

Roger