DLS Course 5 Week4 Ex 8 Failing Grader

In the notebook, all tests pass, but when I submit to the grader, it says exercise 8 fails.

Code Cell UNQ_C1: Function 'get_angles' is correct.
Code Cell UNQ_C2: Function 'positional_encoding' is correct.
Code Cell UNQ_C3: Function 'scaled_dot_product_attention' is correct.
Code Cell UNQ_C4: Function 'EncoderLayer' is correct.
Code Cell UNQ_C5: Function 'Encoder' is correct.
Code Cell UNQ_C6: Function 'DecoderLayer' is correct.
Code Cell UNQ_C7: Function 'Decoder' is correct.
Code Cell UNQ_C8: Unexpected error (ValueError('The first argument to `Layer.call` must always be passed.')) occurred during function check. We expected function `Transformer` to return type <class 'tuple'>. Please check that this function is defined properly. 
If you see many functions being marked as incorrect, try to trace back your steps & identify if there is an incorrect function that is being used in other steps.
This dependency may be the cause of the errors.

Please help me find the error.

(edited my reply and added some extra characters to beat the 20-character minimum)