problem with number

my xsl file:
<xsl:variable name="vlevel" select="10">
-----------------------
<xsl:variable name="ind">
<xsl:choose>
<xsl:when test="$vtype='num'">$vlevel + 5</xsl:when>
<xsl:otherwise>$vlevel + 3</xsl:otherwise>
</xsl:choose>
</xsl:variable>


<xsl:value-of select="number($ind)"/>

my output file i have:

NaN

i should have 13 or 15
thanks


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