Docbook+Ant: Could not find variable with the name of fop.extensions

After a lot of spent time trying to get my article to compile in Ant with Docbook, I can't seem to make FO compilation work. I'm using Xalan 2.7.0, and everything else (both single-page and chunked HTML) compiles perfectly. It's only when I try to compile to FO that I get this error:

Fatal Error! org.apache.xml.utils.WrappedRuntimeException: Could not find variable with the name of fop.extensions Cause: org.apache.xml.utils.WrappedRuntimeException: Could not find variable with the name of fop.extensions

This is pretty strange and I can't seem to resolve it. I even added a <param> value defining the variable it "can't find:"

<xslt style="docbook-xsl/fo/fo.xsl" in="documents/book.xml"
            out="output.fo">
    <classpath>
        <fileset dir="lib" includes="**/*"/>
    </classpath>
    <param name="fop.extensi开发者_运维百科ons" expression="1"/>
</xslt>

Is there anything I can do to resolve this issue? It's really weird if you ask me. (Again, using the same code as above, all of my other Docbook compilation works just fine)

Instead of using fo/fo.xsl, try fo/docbook.xsl. That is the main stylesheet for XSL-FO output.