Flatten an array of arbitrarily nested arrays of integers

flatize.rb

#! /usr/bin/env ruby
def flatize(array)
  error_msg = 'Method only accepts nested arrays of integers'
  raise ArgumentError, error_msg unless array.is_a? Array
  flattened_array = []
  array.each do |element|
    if element.is_a? Array 
     flattened_array += flatize(element)
    else
     raise ArgumentError, error_msg unless element.is_a? Integer
     flattened_array << element
    end
  end
  return flattened_array
end

flatize_test.rb

#! /usr/bin/env ruby
require 'minitest/autorun'
require_relative 'flatize.rb'
class FlatTest < Minitest::Test
  def test_simple
    assert_equal [1,2,3,4], flatize([[1,2,[3]],4])
  end
  def test_deep_nest
    assert_equal [1,2,3,5,14], flatize([[1,2,[3]],[],[[5,[[[14]]]]]])
  end
  def test_bignum
    assert_equal [4,6,98,11111111111111111111], flatize([4,6,98,[[11111111111111111111]]])
  end
  def test_empty_param
    assert_equal [], flatize([])
  end
  def test_invalid_array_elements
    assert_raises(ArgumentError) {flatize(['foo',0.65,nil])}
  end
  def test_non_array_param
    assert_raises(ArgumentError) {flatize('bar')}
  end
end