numbering of book parts

I’d like to use uppercase roman numerals for part
numbers, which should be the default, but instead DocBook is using arabic numerals.
Is this a defect in the stylesheet? It looks like fo\division.xsl correctly
specifies a roman numeral for part numbers, but it’s not working:

 

<xsl:template match="part" mode="division.number">

  <xsl:number from="book" count="part" format="I."/>

</xsl:template>

 

Toolchain:

DocBook XSL 1.7.2.0

Saxon 6.5.3

FOP 0.93

 

Regards,

Jeff Powanda