diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index 55d2212a8a..d0875adce0 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Document :allow_nil option for validates_acceptance_of since it defaults to true. [tzaharia]
+
 * Update documentation for :dependent declaration so that it explicitly uses the non-deprecated API. [danger]
 
 * Add documentation caveat about when to use count_by_sql. [fearoffish]
diff --git a/activerecord/lib/active_record/validations.rb b/activerecord/lib/active_record/validations.rb
index 296b486023..4362e08f3a 100755
--- a/activerecord/lib/active_record/validations.rb
+++ b/activerecord/lib/active_record/validations.rb
@@ -414,6 +414,7 @@ module ActiveRecord
       # Configuration options:
       # * <tt>message</tt> - A custom error message (default is: "must be accepted")
       # * <tt>on</tt> - Specifies when this validation is active (default is :save, other options :create, :update)
+      # * <tt>allow_nil</tt> - Skip validation if attribute is nil. (default is true)
       # * <tt>accept</tt> - Specifies value that is considered accepted.  The default value is a string "1", which
       # makes it easy to relate to an HTML checkbox.
       # * <tt>if</tt> - Specifies a method, proc or string to call to determine if the validation should