merge job failing on signing template pipeline

I have a private pipeline with signing templates with one of the signed source merge jobs failing oddly:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/debusine/tasks/_task.py", line 864, in _execute
    execution_succeeded = self.run(execute_directory)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/tasks/mergeuploads.py", line 296, in run
    merged_changes_path = self.merge_changes(execute_directory, all_changes)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/tasks/mergeuploads.py", line 274, in merge_changes
    merged.order_before("Binary", "Source")
  File "/usr/lib/python3/dist-packages/debian/deb822.py", line 552, in order_before
    self.__keys.order_before(_strI(field), _strI(reference_field))
  File "/usr/lib/python3/dist-packages/debian/_util.py", line 370, in order_before
    self._reorder(item, lambda x: self.__order.insert_before(x, reference_node))
  File "/usr/lib/python3/dist-packages/debian/_util.py", line 385, in _reorder
    node = self.__table[item]
           ~~~~~~~~~~~~^^^^^^
KeyError: 'Binary'

/cc @cjwatson