xsl:function problem

I'm using a very simple function and transforming through oxygen
configured to use Saxon8B.

The function seems to be ok when no parameters are required, but seems
to ignore xsl:param when added?

<xsl:function name="p2f:lookupNameFor">
       <xsl:param name="formOID" as="xs:string"/>
       <xsl:value-of
select="tokenize(//ItemGroupDef[OID=$formOID]/@Name, '::')"/>
</xsl:function>

<xsl:variable name="identifiers"><xsl:value-of
select="p2f:lookupNameFor('test')"/></xsl:variable>

<xsl:variable name="identifiers"><xsl:value-of
select="p2f:lookupNameFor('test')"/></xsl:variable>

F [Saxon8B] XPath syntax error at char 30 on line 23 in
{p2f:lookupPrismNameFor('test')}:
   Cannot find a matching 1-argument function named
{http://www.nextrials.com/ns/prism2flex/v1.0}lookupNameFor()