So what this means is the class 5 is not a part of your training set, but is in the validation. You may need to either oversample your data or go a bit further with the DataBlock API. We can “fix” the problem by passing a list of classes, however if your data for 5 is so rare that it’s only showing up in the validation set, it may be better to drop all instances of class 5 instead
(I’m also wondering why when you built the DataLoaders an exception wasn’t raised stating this…)