Error Message: the operation length of list cannot accept the arguments

I have a student trying to make an app that features several buttons that will allow the user to answer trivia type questions. He keeps getting the error message shown above.

Any suggestions to fix this?


The displayed blocks in the background do not include a Length of List block.

Hunt down where that block is used, and for each instance of that block trace back its list input and insure it is actually a list.

Posting blocks images wouldn't hurt, too.