xsl:function, arithmetic

Hello,

I'm having a few problems with an xsl:function.

Its supposed to take a numerical parameter and return it multiplied by
1.2 and rounded:

<xsl:function name="my:increase">
    <xsl:param name="i" />
    <xsl:value-of select="round(1.2 * $i)" />
</xsl:function>

I've tried added an xsl:message to the function to check the XPath
expression and it seems to be correct, but all the expressions in my
stylesheet which use this function seem to return non-numerical results.
I've also tried just returning the parameter unaltered, in which case I
get no problems. And I've tried things like 'select="$i + $i div 5"' but
still get the problem.

I'm using XSLT2, Saxon 8.2B.

Any ideas?

Richard