Test param value type in XSL - xslt

Question

I am getting this from LibXSLT:
XSLTProcessor::transformToXml(): Invalid type
XSLTProcessor::transformToXml(): xmlXPathCompiledEval: 1 objects left on the stack.
I am passing a param which can either have a string value or a nodeset. I am trying to test whether it contains a certain substring and in that case assign that value to another parameter.
The calling template:
<xsl:call-template name="img">
<xsl:with-param name="upload" select="'url.com/image.jpg'"/>
<xsl:with-param name="w" select="200"/>
<xsl:with-param name="h" select="200"/>
</xsl:call-template>
The called template:
<xsl:template name="img" match="*" mode="w">
<xsl:param name="upload" select="."/>
<xsl:param name="JITexternal">
<xsl:choose>
<xsl:when test="
not($upload/meta) and (contains($upload, '.jpg')
or
contains($upload, '.png'))
">
<xsl:value-of select="$upload"/>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="0"/>
</xsl:otherwise>
</xsl:choose>
</xsl:param>
</xsl:template>
While I am not sure what is tripping LibXSLT, I think is that fact that when I run those tests and the value is a string it throws the type error above.
But most importantly, is there a good way to test the type of a param's value?
UPDATE: the full XSL script on GitHub

Answer 1

While not exactly type checking, I found that converting the result tree fragment to a string() before running my test did prevent LibXSLT to bomb out:
<xsl:param name="JITexternal">
<xsl:choose>
<xsl:when test="starts-with(string($upload), 'http://')">
<xsl:value-of select="$upload"/>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="0"/>
</xsl:otherwise>
</xsl:choose>
</xsl:param>