fm2019 & glossgroup

Good time of day!

My bookmap is looking like:

<bookmap xml:lang="en-GB">
  <part ...>
    <chapter ...>
       ...
    </chapter>
  </part>
  <backmatter>
    <booklists>
      <glossarylist navtitle="Glossary" id="gloss">
        ...
        <topicref href="glossary/ena.dita" id="ena">  <=== This file is <glossgroup>
          ...
          <keydef href="glossary/ena.dita#aecma" keys="aecma"/>  <=== FM swears: "no file with such name"
          ...
        </topicref>
        ...
      </glossarylist>
      <indexlist/>
    </booklists>
  </backmatter>
</bookmap>

What am I doing wrong?