The body of the test is:

 <xsl:template name="main">
    <out>
       <x y="f:f()"/>
    </out>
 </xsl:template>

but this doesn't call the function. I think that the intention of the test is to raise the error dynamically, though it could statically be detected, but if the function is never called, the error does not need to be raised.

Proposed fix: change the attribute value into an AVT.