To draw an analogy with more mainstream type systems, say you have a function like add(x: Int, y: Int): Int (takes two integers and returns an integer).

Now, somewhere else in your code, you have something like:

x = readline()

y = readline()

add(x, y)

That won't compile because readline returns string and add expects integers. Somewhere between readline and add you'll have to convert the strings to integers, and that piece of code (not the add function) is the one that has to be concerned with things like the user entering "abc" where a number is expected.

As far as the add function itself is concerned, it will always be invoked with two integers; the burden of proof is on the caller.