Why do associative arrays throw an Error (RangeError) on value not found?

[Supernova]

Why do associative arrays throw an Error (RangeError) on value 
not found?

This seems like it would be inefficient to check for, so a 
recoverable Exception (ItemNotFoundException?) would seem to be 
more appropriate.

Or is Array[Key] with a key that is not in the array and bounds 
checking disabled undefined behavior?