Sometimes it is necessary to raise an exception. When you do this, you should always include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. For situations where you know that the error source will be a certain type, you can choose to raise one of the built in error types, but should still include a meaningful message.
This particular exercise requires that you refactor how/where the raise statement is used to "throw" a ValueError for invalid tree input. The tests will only pass if the code both raises the appropriate exception and includes an appropriate message with it.