Question using xsl:if and xsl:param

I am trying to do a xsl:if with a value that was assigned using xsl:param.
How do I reference it...here is what I have tried some far using Xalan.

<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xsl:param name="SHOW_IMAGES"/>
  <xsl:template match="/">
  <xsl:if test="$SHOW_IMAGES == 'TRUE'">
    <!-- Do something -->
  </xsl:if>
</xsl>

Here is the error I get.
(Location of error unknown)XSLT Error
(javax.xml.transform.TransformerConfigurationException):
javax.xml.transform.TransformerException: java.lang.RuntimeException:
Programmer's assertion in getNextStepPos: unknown stepType: -1

any ideas??
Thanks
Chris

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list