"Just don't do that" is the correct approach to errors, even when they are easy to overlook and the programming language provides many opportunities for mistakes.

For example, you seem to underestimate how wrong placing negative values in a signed char is: ordinary character encodings do not use negative codes, so either those negative values are not characters and they have no business being treated as such, or something strange and experimental is going on.

Just don't fall bro. It's that easy. No railings required.