Flatten an array of nest arrays

flatten.rb

def flatten(array)
  return "Input must be an array" if !array.kind_of?(Array)
  array.reduce([]) do |res, el|
    Array === el ? res + flatten(el) : res << el
  end
end

flatten_spec

require 'flatten'

describe '#flatten' do
  it "must raise an error if the input is not an array" do
    input = "string"
    expect(flatten(input)).to eq("Input must be an array")
  end

  it "must raise an error if the input is null" do
    input = nil
    expect(flatten(input)).to eq("Input must be an array")
  end

  it "must return an empty array if it is given an empty array as an input" do
    empty_array = []
    expect(flatten(empty_array)).to eq([])
  end

  it "must flatten an array of nested integers" do
    array = [1,2,[3,[4,5]],[6,[7]],[8,9,10]]
    expect(flatten(array)).to eq([1,2,3,4,5,6,7,8,9,10])
  end
end