So, I don't really know what the error is, because it says that the program took too long to finish, and that I should check for infinite loops.
$VERBOSE = nil # We'll explain this at the end of the lesson.
require 'prime' # This is a module. We'll cover these soon!
unless n.is_a? Integer
return "n must be an integer."
if n <= 0
return "n must be greater than 0."
prime_array ||= 
prime = Prime.new
for num in (1..n)
So, the only thing I changed was the prime_array ||=  and I don't see where the problem could be.
I tried again, and it worked.