using xsl:message

I have a required top-level parameter in my stylesheet:

<xsl:param name="citation-style" required="yes" as="xs:string" />

Currently, if that's  missing, I get this error:

  Circular definition of variable citation-style
Transformation failed: Run-time errors were reported

... which is not helpful.

Can I somehow add a message to the parameter to to return if it's not 
specified?  It's used in a variety of places.
Bruce