invalid xpath expression

my xsl file is:

<xsl:choose>
<xsl:when test=".=list">
<xsl:variable name="n" select="12"/>
</xsl:when>
<xsl:otherwise>
<xsl:variable name="n" select="2"/>
</xsl:otherwise>
</xsl:choose>
-------------------
<xsl:variable name="m" select="$n"/>

i receive the error :invalid xpath expression!
so what the problem in <xsl:variable name="m" select="$n"/>?


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