There was a problem compiling the code from your notebook. Details: schema is of type NoneType, should be a Schema proto

Hello. I would really appreciate some help with an error that I am getting.
In W3 graded assignment I get all the expected outputs with my code, however when it comes to the grading, the grader outputs the following error message for my submissions :

There was a problem compiling the code from your notebook. Details:
schema is of type NoneType, should be a Schema proto.

I do not understand this error and would appreciate your help with this.

If you’ve completed all the exercises, then, please click my name and message your notebook as an attachment.

If that’s not the case, please search online for #grade-up-to-here and use it for partial regrading of the assignment.

Thank you, Balaji.
I was able to solve it by completing the last exercise.
Best,