C :: Function Return Code / Value?

I have a function that needs to return a "uint8_t" value. However before doing the processing I need to perform a test on the argument to check if it's between expected boundaries. Although this function works it gives (a logical) warning that not always a value is returned although expected. What is the normal way for functions like these where I normally should return e.g. -1 in case the test doesn't succeed and otherwise the uint8_t (t) value?

Code:
uint8_t myFunc(int a) {
if (a >= 0 && a <= 100) {
// Perform actions
uint8_t = ...
return t;
}
}