xsl:if giving me "Invalid type" error

<xsl:if test="$firstDiv[1]/preceding-sibling::*">
 ...
</xsl:if>

only works if $firstDiv[1]/preceding-sibling::*
returns empty. Otherwise, I get a complaint about

XPath error Invalid type in
$firstDiv[1]/preceding-sibling::*

Nothing I can think of seems to fix this. I tried
using boolean($firstDiv[1]/preceding-sibling::*), but
I still get the Invalid type error.

What's going on?


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