What is your ciphertext and where do you add it in your code?

The error message is raised from the ->crypt method call. You don't show that call.

Please go back and rewrite your code to a small, self-contained program that reproduces the problem. Please also show all needed input data to reproduce the problem.

Most likely, the problem is that your ciphertext that you are trying to decode does not begin with a valid header. This is more a data problem than a problem in your program.