Different number of classes in training and validation data

Part 1 (2017)
1

Error when checking model target: expected dense_8
I get the error above when I run the following code:

Screen Shot 2017-07-28 at 6.41.51 PM.png2054×406 67.8 KB
Screen Shot 2017-07-28 at 6.42.07 PM.png2230×796 143 KB

It seems to me that since the validation data contains not all the classes present in the training data, something breaks. Is it necessary that the validation data must contain data for all classes in the training data?

Thanks a lot for the help!