Max of two natural numbers

Write a function with the following type:

val max_nat : int -> int -> int

such that max_nat a b evaluates to the maximum between a and b, if they are both natural numbers. If not, the function gives an error.

For instance, if the first argument is negative, we will obtain something like:

max_nat (-3) 5 ;;
Exception: "Assert_failure //toplevel//:2:2"

Warning: do not even think of using the built-in max function.