Exercise "Simple Cipher" random key tests pass with constant key

The exercise definition requires a random key to be generated if no other key is given. However, I’ve found that setting a constant key of sufficent length (>= 10) e.g. Key=“ddddddddddddddd” passes all tests.
Apparently, there’s a test to check if the ‘random’ key consists only of lower-case letters, but none to check wether it is actually random or, at least, non-constant.