"a malicious user (or just a very ignorant one) can enter inputs that are so ambiguous"

Have you considered introducing restrictions on what/how a user can enter input by ruling out which input is considered valid and which one is considered otherwise?.

"the error will happen when it's just adding an element to a pre-existing hash"

show a more detailed code example that represents this problem, without that example it is just not easy to pinpoint where the issues lie and we'd be only trying to make informed guesses, but they just remain 'guesses' until proven..