calendar problems with two drivers

IPSL-IPSL-CM5A-MR MOHC-HadGEM2-ES

Datasets containing one of those drivers throw the following exception / problem: "[GenerateStatistics] ... for entire concatinated period [1971-01-02] - [2100-12-31] Only valid with DatetimeIndex, TimedeltaIndex or PeriodIndex, but got an instance of 'Index' "