inputString returns blank for non-char keys

I'm designing a cheat code system similar to Doom or Contra where the player must enter a sequence of inputs in the correct order. I've done this by creating a string array for the key names of required inputs and an index counter to move through them. If the wrong key is pressed at any time, the index resets to 0.

This mostly works except for the fact that any non-char key, such as the arrows keys, spacebar, return, etc. doesn't send a name in its string through inputString. Am I missing something or are there specific key codes needed?