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)

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