How to fix a NoMethodError :undefined method `<<' for nil:NilClass - ruby

Question

I have 3 error messages which I believe all relate to one thing - me calling a method on an object that has not been defined properly.
I've checked over for typos, white spacing, syntax, naming. I'm following the same process as other methods in this program which all tested OK.
I've narrowed the loop down and believe the issue is here:
def add(param)
#param << param
Heres the code in full context:
class Todo
def initialize(param)
#param = param
end
def text
return #param
end
end
class TodoList
def initialize
#item_list = []
end
def add(param)
#param << param
end
def print
#item_list.each do |param|
puts "* #{param.text}" #=> .text?
end
end
end
Error Messages:
Here are the error messages I'm trying to resolve. As I mentioned earlier they all look to be pointing to a method I've not defined properly but I've not yet been able to work out why that may be.
1) Q1. Todo list TodoList adding todos adds a todo without raising an error
Failure/Error: expect { todo_list.add(todo) }.to_not raise_error
expected no Exception, got #<NoMethodError: undefined method `<<' for nil:NilClass> with backtrace:
# ./questions/question_1.rb:43:in `add'
# ./spec/question_1_spec.rb:31:in `block (5 levels) in <top (required)>'
# ./spec/question_1_spec.rb:31:in `block (4 levels) in <top (required)>'
# ./spec/question_1_spec.rb:31:in `block (4 levels) in <top (required)>'
2) Q1. Todo list TodoList printing todos one todo prints a single todo with a bullet point
Failure/Error: todo_list.add(todo)
NoMethodError:
undefined method `<<' for nil:NilClass
# ./questions/question_1.rb:43:in `add'
# ./spec/question_1_spec.rb:41:in `block (5 levels) in <top (required)>'
3) Q1. Todo list TodoList printing todos many todos prints the todos, separated by newlines
Failure/Error: todo_list.add(todo_1)
NoMethodError:
undefined method `<<' for nil:NilClass
# ./questions/question_1.rb:43:in `add'
# ./spec/question_1_spec.rb:59:in `block (5 levels) in <top (required)>'
Thanks!

Answer 1

ruby do not know what #param is in your TodoList class, you have to initialize it in your constructor like this #param = [] that fix your issue

Answer 2

By default instance variables are initialized to nil, hence the error. So in your TodoList class, you need to initialize #param to an object that can respond to <<, which in your own case, is an array []
class TodoList
def initialize
#param = []
end
end