Why does the XTSE1015 Error occur when sorting XML document

I'm looking for some help with an XSL error. It's called XTSE1015 which stands for "An xsl:sort element with a select attribute must be empty". It happens when i try to output file to xml or text. Software such as Oxygen XML Editor or event Notepad++ show this error. I can't find any solution to this, well except not using sort but that is not a proper solution. Somebody stumbled upon this problem maybe and has some advice or something?

<xsl:variable name="strName">
   <xsl:for-each select="Samochody/Model/Nazwa">
      <xsl:sort select="string-length(.)" data-type="number">
          <xsl:value-of select="."/>
       </xsl:sort>
   </xsl:for-each>
</xsl:variable>

Solution 1:

The <xsl:sort... /> element is not allowed to have any children. Hence the error. See, for example, here at Tutorialspoint. So, to make it work, change your code to

<xsl:variable name="strName">
   <xsl:for-each select="Samochody/Model/Nazwa">
      <xsl:sort select="string-length(.)" data-type="number" />
      <xsl:value-of select="."/>
   </xsl:for-each>
</xsl:variable>