Gotcha: NoMethodError: undefined method RuntimeError

So... I wrote some code in a plain-ruby class that raised a RuntimeError somewhat like below:

    class MyClass
      def initialize(opts = {})
         # do stuff
         thing = opts[:thing]
         raise RuntimeError "must have a thing!" unless thing.present? && thing.is_a?(Thing)
         # more stuff
      end
    end

and when I ran my fresh new rspec spec over it -> which looks somewhat like:

    it "should raise an error if we don't pass a thing" do
      lambda {
        my_class = MyClass.new(:thing => nil)
      }.should raise_exception(RuntimeError)
    end

I kept getting something weird:

    expected RuntimeError, got 
    #<NoMethodError: undefined method `RuntimeError' for #<MyClass:0xb5acbf9c>>

You may already have spotted the problem... ah, single-character bugs, doncha love em?

Here it is.

WRONG:

     raise RuntimeError "must have a thing!" unless thing.present? && thing.is_a?(Thing)

RIGHT:

     raise RuntimeError, "must have a thing!" unless thing.present? && thing.is_a?(Thing)

of course, you can also just go ahead and leave out the RuntimeError entirely:

     raise "must have a thing!" unless thing.present? && thing.is_a?(Thing)

because it's the default anyways... which makes it nice and defaultish for you.