diff --git a/activemodel/lib/active_model/validations/length.rb b/activemodel/lib/active_model/validations/length.rb
index 3e6f6e62e6e08ff47e2fb7b4fab0a01f2c344232..72735cfb89f1ae770442068303e150bf0a09258c 100644
--- a/activemodel/lib/active_model/validations/length.rb
+++ b/activemodel/lib/active_model/validations/length.rb
@@ -42,9 +42,9 @@ def validate_each(record, attribute, value)
           next unless check_value = options[key]
 
           value ||= [] if key == :maximum
-          
-          next if value.kind_of?(Fixnum) && value.to_s.size.send(validity_check, check_value)
-          next if value && value.size.send(validity_check, check_value)
+
+          value_length = value.respond_to?(:length) ? value.length : value.to_s.length
+          next if value_length.send(validity_check, check_value)
 
           errors_options = options.except(*RESERVED_OPTIONS)
           errors_options[:count] = check_value
diff --git a/activemodel/test/cases/validations/length_validation_test.rb b/activemodel/test/cases/validations/length_validation_test.rb
index 5e4f4f5cae5ea84bfd32d2f3b22f2e906c0a1b2d..f02514639b00585fb9f1b957a29e6836d6d75bfb 100644
--- a/activemodel/test/cases/validations/length_validation_test.rb
+++ b/activemodel/test/cases/validations/length_validation_test.rb
@@ -341,14 +341,14 @@ def test_validates_length_of_with_block
     assert t.errors[:content].any?
     assert_equal ["Your essay must be at least 5 words."], t.errors[:content]
   end
-  
+
   def test_validates_length_of_for_fixnum
     Topic.validates_length_of(:approved, :is => 4)
-    
+
     t = Topic.new("title" => "uhohuhoh", "content" => "whatever", :approved => 1)
     assert t.invalid?
     assert t.errors[:approved].any?
-    
+
     t = Topic.new("title" => "uhohuhoh", "content" => "whatever", :approved => 1234)
     assert t.valid?
   end