diff --git a/package/MDAnalysis/core/groups.py b/package/MDAnalysis/core/groups.py index b6887d0299d..50f447e5127 100644 --- a/package/MDAnalysis/core/groups.py +++ b/package/MDAnalysis/core/groups.py @@ -569,7 +569,9 @@ def __getitem__(self, item): # because our _ix attribute is a numpy array # it can be sliced by all of these already, # so just return ourselves sliced by the item - if isinstance(item, numbers.Integral): + if item is None: + raise TypeError('None cannot be used to index a group.') + elif isinstance(item, numbers.Integral): return self.level.singular(self.ix[item], self.universe) else: if isinstance(item, list) and item: # check for empty list