Error in XPath 2.0 expression Too many items

Hi,

Can anybody help me in finding out what this error is for ?

I am using 'Altova xmlspy 2006 Enterprise Edition version 2006 sp2' to
convert a source xml to target xml using XSLT and in my .xsl program I
am importing a schema as
<xsl:import-schema  schema-location="abc.xsd"/>

This abc.xsd is importing other schemas.

Now my problem is when I see the 'browser'  present in this xmlspy I
can see values fetched but when I click on 'XSL Transformation' it
gives me an error
"XSL transformation failed due to following error:
Error in XPath 2.0 expression Too many items"

and the cursor stops at this line
<xsl:variable name="city" select="substring-before($vCityStateZip, ',')"/>

I tried to change the place of this line but after error the cursor
points at this line.

Is there any funny mistake I am doing or there is some version problem
or something else ??

Pls suggest. Thanks in advance

Cheers,
Rob