diff --git a/app/models/custom_field.rb b/app/models/custom_field.rb
index 34b9fb53d..055516c38 100644
--- a/app/models/custom_field.rb
+++ b/app/models/custom_field.rb
@@ -26,7 +26,7 @@ class CustomField < ActiveRecord::Base
   validates_presence_of :name, :field_format
   validates_uniqueness_of :name, :scope => :type
   validates_length_of :name, :maximum => 30
-  validates_inclusion_of :field_format, :in => Redmine::CustomFieldFormat.available_formats
+  validates_inclusion_of :field_format, :in => Proc.new { Redmine::CustomFieldFormat.available_formats }
   validate :validate_custom_field
 
   before_validation :set_searchable
diff --git a/lib/redmine/custom_field_format.rb b/lib/redmine/custom_field_format.rb
index f7cd5d1c6..c539aee2e 100644
--- a/lib/redmine/custom_field_format.rb
+++ b/lib/redmine/custom_field_format.rb
@@ -70,6 +70,13 @@ module Redmine
         @@available[custom_field_format.name] = custom_field_format unless @@available.keys.include?(custom_field_format.name)
       end
 
+      def delete(format)
+        if format.is_a?(Redmine::CustomFieldFormat)
+          format = format.name
+        end
+        @@available.delete(format)
+      end
+
       def available_formats
         @@available.keys
       end
diff --git a/test/unit/custom_field_test.rb b/test/unit/custom_field_test.rb
index 051853abc..b99fb4353 100644
--- a/test/unit/custom_field_test.rb
+++ b/test/unit/custom_field_test.rb
@@ -57,6 +57,20 @@ class CustomFieldTest < ActiveSupport::TestCase
     assert field.valid?
   end
 
+  def test_field_format_should_be_validated
+    field = CustomField.new(:name => 'Test', :field_format => 'foo')
+    assert !field.valid?
+  end
+
+  def test_field_format_validation_should_accept_formats_added_at_runtime
+    Redmine::CustomFieldFormat.register 'foobar'
+
+    field = CustomField.new(:name => 'Some Custom Field', :field_format => 'foobar')
+    assert field.valid?, 'field should be valid'
+  ensure
+    Redmine::CustomFieldFormat.delete 'foobar'
+  end
+
   def test_should_not_change_field_format_of_existing_custom_field
     field = CustomField.find(1)
     field.field_format = 'int'