Run time error in Caesar Cipher Code

I'm currently working a Caesar Cipher Program in App Inventor. Every time I try to encrypt or decrypt a message put in the program, I'm getting a run-time error. I think it has something to do with the shift, but I'm unsure and I don't know how to fix it. Any help? I've attached photos

The AI2 text segment block is very fussy about running off the end of its input.

There is a fancy text block that does a replace with dictionary all in one shot.

Build a new dictionary with keys and values shifted the required amount, and use that for the encoding.

That will break down the problem into two smaller parts.

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You appear to be attempting to port a Caesar Cipher algorithm to App Inventor. To do that, a developer needs to know what a Caesar Cipher is. Here is some help

You might do a google search to help you do your homework and find this Caesar Cipher Tutorial - YouTube
or caesar cipher app inventor - Google Search to find your 'error'.