XALAN 1.2 Variable not defined?

[Lucio Tarantino <lu...@volendo.com>]

Hello,
    I have a little problem. In my code I use a lot of imported xsl .
    When I import my file A.xsl some variable can be defined  other must
be set to default.
    I tried code like:

 <xsl:variable name="_navTopImg">
        <xsl:choose>
               <xsl:when
test="not($_navTopImg)">registrazione</xsl:when>
               <xsl:otherwise><xsl:value-of
select="$_navTopImg"/></xsl:otherwise>
        </xsl:choose>
</xsl:variable>

but when the _navTopImg is not defined I receive the follow error:
   VariableReference given for variable out of context or without
definition!  Name = _navTopImg, source tree node: #document.

Please help me! All the code work with XALAN 1.2D02 and I don't have the
time to rewrite all! :-(

Best regards
Lucio