bank_account.rb no method error for '+'

I am working on the Ruby Deep Dive and am having an issue when I try to run the deposit or withdraw functions. I've reinstalled rvm/brew and rewritten the code about a dozen times but when I run

"class BankAccount def initialize(first_name, last_name) @balance = 0 @first_name = first_name @last_name = last_name end

def deposit(amount) @balance += amount end

def withdraw(amount) @balance -= amount end end"

I receive the error: "NoMethodError: undefined method +' for nil:NilClass from /Users/rorymccarron/Source/methods/bank_account.rb:8:indeposit' from (irb):5 from /Users/rorymccarron/.rvm/rubies/ruby-1.9.3-p362/bin/irb:16:in `<main>'"

any suggestions would be appreciated.

Thanks, R