Need help on lisp.

Create a recursive function ALPHABET that takes a list of integers in the 1-26 range. It returns a list of the same length with the corresponding alphabet letter substituted for the integer. Do error checking on the input. Return the empty list if the list is already empty.

[42]> (alphabet '(1 2 26))

(a b z)

[43]> (alphabet '())

NIL

[44]> (alphabet "foo")

Error in ALPHABET function. Parameter must be a list, but was given "foo"

NIL