Check for 'null' doesn't work

Hello!

We've experienced a problem in Workbench regarding Boolean functions within XPath expressions. We aren't able to check whether a variable is null or not. An example:

>Process variable name:
b xy

>Process variable type:
b String

>Process variable value:
b null

>Xpath evaluation:
b is-null(/process_data/@xy) = true()

>Result:
b the Xpath evaluation returns false!!

Does anybody see the problem?
Thanks,
Nico