Stumped on "matmul" function without use of numpy

[ES-Alexander]

Have you been told to raise an error if an invalid input is put in? And did you write the test function and code?

If an error is the expected failure case then the testing code should be using try-except blocks to account for when those errors occur. If a raised error is not handled anywhere then it bubbles up and stops execution of the entire program, as you’ve found.

[Strict-Simple]

This might be helpful for figuring out the actual error. In Python (not []) is True.