Unexplained empty ref attributes in qhp files

'make docs', i.e. running qhelpgenerator, yields ~180 warnings about 'Missing attribute in keyword at line NNN.' (Previously, we only had a couple). This is because .qhp files have a lot of lines like:

<section ref="" title="C++ Classes">
and
<keyword name="QtCore" id="QtCore" ref=""/>

That is, empty ref attributes. To me it seems that qdoc is now generating more keywords than previously, and cannot find ref pages for them. Missing section refs are also problematic: For example, open qtcore.qch in assistant, and select 'C++ Classes' in the left-hard contents view -> 'The page could not be found'.