Ruby 'is_a?' class or module required (TypeError)

Question

I'm playing around with Ruby and trying create a little bank account program. When I run this particular line of code which is running withing the create_account:

unless @response.is_a? Integer && @response.to_str.length == 4     puts "Your response must be 4 numbers in length."     create_account else     @pin = @response     puts "Your pin has been set." end 

I get this response:

bank_account.rb:24:in 'is_a?':class or module required (TypeError)     from bank_account.rb:24:in 'create_account'     from bank_account.rb:47:in '<main>' 

I can't exactly figure out what's going on, but I'm including the rest of my code. It's incomplete, obviously because I'm stuck on this part. I go through a while loop and enter 'Create Account' to start the create_account method. My goal is to run the conditional and require the user to type a 4 digit integer, like any ole pin number. If it is not an integer nor 4 digits in length, I call the create_account method to get them to start over.

I'm running ruby 2.0.0 if that information is necessary, but I wager it is likely more to do with my code. Stackoverflow is new to me, so I apologize if this question has been asked. I tried to do my homework as is advised before asking a question, but I'm still stumped. Any help is appreciated.