xsl variable - issue

Hello,
   Please take a look at the XML and XSL files. I have
variable called contentsVar which contains a node set.
For some reason, the 'if' statement is failing though
the XML says isvisible = true. Not sure what I am
doing wrong. 
Any help will be appreciated.

Thanks.

XML:

<root>
   <book>
      <contents isvisible="true">
          <chap>
          <chap>
       </contents>
   </book>
</root>

XSL:

<xsl:template match="book">
<xsl:variable name="contentsVar" select="contents"/>
.
.
.
<xsl:if test="$contentsVar/@isvisible = 'true'">
do something...
</xsl:if>
</xsl:template>

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