.substring error

My students are working on the Unit 7 Build a Library project. One of my students is attempting to use the built-in .substring function. App Lab keeps giving us the error: “TyperError: str.substring is not a function” - It worked on Lesson 7 in the Pigify app library. We are attempting to use it in almost exactly the same way. We tried pulling it from the toolbox, copying and pasting the code from the Pigify app, and typing it out ourselves. We get the same error every time. It makes no sense that it would work in the practice level, but then tell him in the next lesson that it isn’t a function.
Any insight?

Can you send us a link to your project so we can look at it? It’s easier to debug that way.