XPath error message (data types)

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XML

Hello,

I do not understand why I get the following error from XPath:

1.
   <xsl:choose>
           <xsl:when test="$rownr = 0">
                   <xsl:attribute name="FontSize" select=" '24 px' "/>
                   <xsl:attribute name="Foreground" select=" 'MidnightBlue' "/>
           </xsl:when>
   </xsl:choose>

   <!-- Warning: Required item type of first operand of '=' is numeric; supplied value has item type xs:string -->

All I understand from it, is that the variable $rownr is supposed to be given a numeric value as right operand. I do not understand that, because $rownr is of an undefined type (I think) and the right operand is already numeric, i.e. 0 and not '0' (I think).

How do I fix this and what is really going on in my code?
I am using EditX Xml Editor free version.

Thank you

//Edit: I have fixed it by writing this:
<xsl:param name="rownr">0</xsl:param>

I do not know why the error was fixed this way.
Can someone please tell me?

Thank you