What is the solution to this blank in the code?

Question

This is from the test_symbols_cannot_be_concatenated exercise in the Ruby Koans. Previous exercises had used assert_equal tests. This is the first assert_raise on the path to enlightenment.

def test_symbols_cannot_be_concatenated

  assert_raise(_____) do
    :cats + :dogs
  end
end

Answer 1

NoMethodError I guess, since it makes no sense to try to sum or catenate two symbols.