Number formatting with <xsl:number />

Hi!
I'm using Xeena 1.1 to create HTML files from XML files.
I'm dealing with a list of <option> elements which should be displayed in 
a table, each row one element. To make it a little more readable, I would 
have to have an index (formatted as a), b), c) ...) for each row. To 
obtain the current index I count the preceding-sibling::option axis. This 
all works pretty well, just when I want to convert the numerical index 
into the corresponding letter, the XSL engine can't parse the stylesheet 
anymore.
The code I'm using looks like this:

<xsl:template match="option" mode="HTML">
<xsl:variable name="rowIndex"><xsl:value-of 
select="count(preceding-sibling::option)"/></xs:variable>
        <TR>
                <TD>
                        <xsl:number value="number($rowIndex+1)" 
format="a"/>
                </TD>
                <TD>
                        further row content
                </TD>
        </TR>
</xsl:template>

The error message is pretty meaningless:
XSL Error: SAX Exception, Location file: option.xsl, line 15, offset 62 
(which is actually the closing angle bracket of the <xsl:number/> tag)

If I replace the 
        <xsl:number value="number($rowIndex+1)" format="a"/>
tag with 
        <xsl:value-of select="number($rowIndex+1)"/>
I get the index I want, just as number and not as a letter. Since I 
already use numbers for a different level of nested list, this would cause 
confusion and I don't want to get into hierarchical numbering, because 
this would make the column pretty wide (besides I would also want to do 
that with <xsl:number> and not manually).
What do I do wrong? Or doesn't the XSLT engine in built-in Xeena support 
<xsl:number>? I thought that was a mandatory element...

Any ideas?

Thanx in advance,
                Ragnar

Attachment: smime.p7s
Description: S/MIME Cryptographic Signature