diff --git a/binary/binary_test.rb b/binary/binary_test.rb
index 03ee5917c9..652eace6ff 100755
--- a/binary/binary_test.rb
+++ b/binary/binary_test.rb
@@ -4,6 +4,10 @@
 require_relative 'binary'
 
 class BinaryTest < Minitest::Test
+  def test_binary_0_is_decimal_0
+    assert_equal 0, Binary.new('0').to_decimal
+  end
+
   def test_binary_1_is_decimal_1
     assert_equal 1, Binary.new('1').to_decimal
   end
@@ -38,13 +42,24 @@ def test_binary_10001101000_is_decimal_1128
     assert_equal 1128, Binary.new('10001101000').to_decimal
   end
 
-  def test_invalid_binary_string_is_decimal_0
+  def test_binary_ignores_leading_zeros
+    skip
+    assert_equal 31, Binary.new('000011111').to_decimal
+  end
+
+  def test_invalid_binary_numbers_raise_an_error
     skip
-    assert_equal 0, Binary.new('carrot123').to_decimal
+    %w(012 10nope nope10).each do |input|
+      assert_raises ArgumentError do
+        Binary.new(input)
+      end
+    end
   end
 
-  def test_invalid_binary_numeric_string_is_decimal_0
+  # This test is for the sake of people providing feedback, so they
+  # know which version of the exercise you are solving.
+  def test_bookkeeping
     skip
-    assert_equal 0, Binary.new('123').to_decimal
+    assert_equal 1, Binary::VERSION
   end
 end
diff --git a/binary/example.rb b/binary/example.rb
index 77b2a3a815..9460a1ba1c 100644
--- a/binary/example.rb
+++ b/binary/example.rb
@@ -1,18 +1,22 @@
 class Binary
+  VERSION = 1
+
   attr_reader :digits
-  def initialize(decimal)
-    @digits = normalize(decimal).reverse.chars.collect(&:to_i)
+  def initialize(s)
+    raise ArgumentError.new("invalid binary input #{s}") unless valid?(s)
+
+    @digits = s.chars.reverse.collect(&:to_i)
   end
 
   def to_decimal
-    digits.each_with_index.inject(0) do |decimal, (digit, index)|
-      decimal + digit * 2**index
+    digits.each_with_index.inject(0) do |decimal, (digit, i)|
+      decimal + digit * 2**i
     end
   end
 
   private
 
-  def normalize(string)
-    string.match(/[^01]/) ? '0' : string
+  def valid?(s)
+    s.chars.all? {|char| ['0', '1'].include?(char)}
   end
 end