RSpec question about catching expected exception in class .new

I have the following I'm trying to test:

  class Marker
    def initialize(secret, guess)
      raise ArgumentError, "secret and guess must be the same length",
caller if guess.length != secret.length
      @secret, @guess = secret, guess
    end

   ...


In my spec test, how do I capture that exception? I'm trying to test
that it raises appropriately when secret and guess are different
lengths.

What I've tried:

      context "unequal lengths" do
        it 'returns "secret and guess must be the same length"' do
          Marker.new('1234','').should raise_error("secret and guess
must be the same length")
        end
      end

But it doesn't actually catch the error, calling it a failed test.
I've also tried raise_error(ArgumentError) with the same result. (I've
also tried calling Maker::new, but that also has the same result.