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Section 15.1 of XSLT 2.0 states that: "If the result of evaluating the select expression is not a string, it is converted to a string by applying the function conversion rules." Should function conversion fail XPTY0004 will be raised. It would be nice if an XTTE error code had been assigned to this condition. Compare with xsl:apply-templates "[ERR XTTE0520] It is a type error if the sequence returned by the select expression contains an item that is not a node."
Decided that this didn't justify a change to the spec, beyond an editorial clarification that the expected code is XPTY0004.
This clarification will be erratum E53.