[xsl] result-document error

["R. Neville" <notanotheracronym@xxxxxxxxx>]

Dear XSL-List,

I have been trying to output multiple documents using result-document
in xsl 2.0, but I keep getting the error "cannot write more than one
document to the same URI."

This is what I have written:

<xsl:result-document href="{concat(pb[@ref], '.xml')}">
<xsl:call-template name="header"/>
</xsl:result-document>

I thought maybe the concat function was the problem, but when I tried this

<xsl:result-document href="pb[@ref].xml">
<xsl:call-template name="header"/>
</xsl:result-document>

I recieved the error "Invalid syntax for base URI."

Kind Regards,
Robert