In an xsl:catch, several variables are automatically bound, to
access error details (err:code, err:description, etc.)[1] The
type of err:description is defined to be xs:string.  What is it
supposed to be bound to in the following case (i.e. when invoking
error() without any description string):

    <xsl:try select="error(QName('ns', 'code'))">
       <xsl:catch>
          <xsl:value-of select="$err:description"/>
       </xsl:catch>
    </xsl:try>

  Note that XQuery 1.1 defines the equivalent variable (in its
own grammar) to be of kind xs:string?, so optional.  My humble
opinion is that XQuery is correct here.