diff --git a/actionpack/lib/action_view/helpers/form_helper.rb b/actionpack/lib/action_view/helpers/form_helper.rb
index 81f6fec8273cde0b4147affbb6c83ffd0f5b31d3..ab387b7bf8f8112acd5e636a7b44ebae073ba069 100644
--- a/actionpack/lib/action_view/helpers/form_helper.rb
+++ b/actionpack/lib/action_view/helpers/form_helper.rb
@@ -121,9 +121,10 @@ module FormHelper
       #   end
       #
       def form_for(object_name, object, options = {}, &proc)
-        concat form_tag(options, options.delete(:html) || {}), proc.binding
+        raise ArgumentError, "Missing block" unless block_given?
+        concat(form_tag(options, options.delete(:html) || {}), proc.binding)
         fields_for(object_name, object, options, &proc)
-        concat '</form>', proc.binding
+        concat('</form>', proc.binding)
       end
 
       # Creates a scope around a specific model object like form_for, but doesn't create the form tags themselves. This makes
@@ -141,10 +142,8 @@ def form_for(object_name, object, options = {}, &proc)
       # Note: This also works for the methods in FormOptionHelper and DateHelper that are designed to work with an object as base.
       # Like collection_select and datetime_select.
       def fields_for(object_name, object, options = {}, &proc)
-        raise ArgumentError, "fields_for requires a block!" unless block_given?
-        builder_klass = options[:builder] || FormBuilder
-        form_builder = builder_klass.new(object_name, object, self, options, proc)
-        yield form_builder
+        raise ArgumentError, "Missing block" unless block_given?
+        yield (options[:builder] || FormBuilder).new(object_name, object, self, options, proc)
       end
 
       # Returns an input tag of the "text" type tailored for accessing a specified attribute (identified by +method+) on an object
diff --git a/actionpack/lib/action_view/helpers/prototype_helper.rb b/actionpack/lib/action_view/helpers/prototype_helper.rb
index 96f065cc110dba3c812448883ef5faaf73e01cee..8aabe04aac34ec2ed517a935b02691e3b90208d8 100644
--- a/actionpack/lib/action_view/helpers/prototype_helper.rb
+++ b/actionpack/lib/action_view/helpers/prototype_helper.rb
@@ -172,9 +172,9 @@ def form_remote_tag(options = {})
 
       # Works like form_remote_tag, but uses form_for semantics.
       def remote_form_for(object_name, object, options = {}, &proc)
-        concat form_remote_tag(options), proc.binding
+        concat(form_remote_tag(options), proc.binding)
         fields_for(object_name, object, options, &proc)
-        concat '</form>', proc.binding
+        concat('</form>', proc.binding)
       end
       alias_method :form_remote_for, :remote_form_for