error using combine_multiple_datasets with powerplantmatching.data.GEM()

Version Checks (indicate both or one)

• I have confirmed this bug exists on the lastest release of powerplantmatching.

• I have confirmed this bug exists on the current master branch of powerplantmatching.

Issue Description

Hi, I’m following a minimal example from the readthedocs but run into an error with the combine_multiple_datasets function that’s traced back to duke.py

Any clues as to why ‘pm.data.GEM()’ may be running into this issue? Thanks!

Reproducible Example

import powerplantmatching as pm
entsoe_ppm = pm.data.ENTSOE()
gem = pm.data.GEM()
dfs = [gem.powerplant.aggregate_units(), entsoe.powerplant.aggregate_units()]
intersection = pm.matching.combine_multiple_datasets(dfs)

Expected Behavior

#error in duke.py
#logger.debug("Comparing files: %s", ", ".join(labels))
#TypeError: sequence item 0: expected str instance, NoneType found

I only get this error in using ‘pm.data.GEM()’. Any other source that I attempt to match with ENSTOe works (JRC, GCPT, GEO, etc)

Installed Versions

Run the minimal example with the documented powerplantmatching release or master version, then inspect combine_multiple_datasets and the referenced duke.py location where the labels are joined. Compare the GEM and ENTSOE inputs and capture the complete traceback and installed versions; done means GEM can be combined with ENTSOE without the TypeError and the regression is covered by a test.