NullPointerException w/ nested includes in RelaxNG compact syntax

pipeline.xpl

<p:declare-step version="1.0" xmlns:p="http://www.w3.org/ns/xproc">
  <p:validate-with-relax-ng>
    <p:input port="source">
      <p:inline>
        <foobar/>
      </p:inline>
    </p:input>
    <p:input port="schema">
      <p:data href="schema0.rnc" content-type="text/plain"/>
    </p:input>
  </p:validate-with-relax-ng>
  <p:sink/>
</p:declare-step>

schema0.rnc

include "schema1.rnc"

schema1.rnc

include "schema2.rnc"

schema2.rnc

start = element foobar { empty }