Great! Thanks for the feedback!

Thanks for the clarification. I'm sorry for the confusion.
I found a bug in my code. It works now :) Code submitted.

There is no test with "aettsT!ih ssi". As you can see this string is not in the test cases.
This error says: You decrypted the text to "aettsT!ih ssi" and not to "This is a test!"

The problem is in your code.
Could you paste your code here? (Mark as spoiler!)

I passed 11 tests and failed 1. My code is in Python 2.
The failed case is this: 'aettsT!ih ssi' should equal 'This is a test!'

There is only 1 space in the text to be decrypted. How can this text be decrypted to 'This is a test!' which has 3 spaces??