`summarycombine` fails with a PESummary metafile

When @john-veitch tried and run summarycombine on multiple PESummary result files, the following error was seen:

Traceback (most recent call last):
  File "/home/john/.local/bin/summarycombine", line 8, in <module>
    sys.exit(main())
  File "/home/john/.local/lib/python3.7/site-packages/pesummary/cli/summarycombine.py", line 39, in main
    args = func["input"](opts)
  File "/home/john/.local/lib/python3.7/site-packages/pesummary/gw/inputs.py", line 655, in __init__
    super(GWInput, self).__init__(opts, ignore_copy=True)
  File "/home/john/.local/lib/python3.7/site-packages/pesummary/core/inputs.py", line 1198, in __init__
    self.samples = self.opts.samples
  File "/home/john/.local/lib/python3.7/site-packages/pesummary/core/inputs.py", line 444, in samples
    self._set_samples(samples)
  File "/home/john/.local/lib/python3.7/site-packages/pesummary/core/inputs.py", line 509, in _set_samples
    "{}/{}".format(key, label), pp[key][label]
KeyError: 'A4-PhenomHM'