> when this line is used:
> 
> <xsl:param name="filter" select="'food'"></xsl:param>
> 
> this error is generated:
> 
> "Exception Details: System.NullReferenceException: Object 
> reference not set 
> to an instance of an object."

I think David was right, you've found a Microsoft bug. Time to change
supplier.

Michael Kay
http://www.saxonica.com/