In MOTAccumulator.merge_event_dataframes （Line 443), we use copy.index.map to update index.
When copy is an empty DataFrame, the size of copy.index is 0.
In this situation, copy.index.map will return an object of type Index rather than MultiIndex.
We can fix it by adding if copy.index.size != 0 before copy.index.map
In
MOTAccumulator.merge_event_dataframes（Line 443), we usecopy.index.mapto update index. Whencopyis an empty DataFrame, the size ofcopy.indexis 0. In this situation,copy.index.mapwill return an object of typeIndexrather thanMultiIndex.We can fix it by adding
if copy.index.size != 0beforecopy.index.map