According to Codewars Kata Authoring Guidelines:

Keep kata focused on one task. Avoid distractions, do not add other, smaller, "side" requirements that do not add too much value to the task itself. Additional "steps" required to solve the task usually do not make the kata more interesting, but are rather seen as tedious and distracting. While things like validation of the input, parsing or stringification, filtering of invalid elements, etc. can be a part of some kata related to specific topics (for example, parsers or interpreters), usually they are just additional nuisance a user has to struggle with, and do not add too much value to the task.

Input validation (like the data type being other than string and number) for a kata that focuses on encryption is pretty irrelevant to the task itself and shouldn't be added to begin with.