diff --git a/lib/redmine/field_format.rb b/lib/redmine/field_format.rb
index 5e81055f1..a6c5d08a2 100644
--- a/lib/redmine/field_format.rb
+++ b/lib/redmine/field_format.rb
@@ -829,6 +829,8 @@ module Redmine
     end
 
     class EnumerationFormat < RecordList
+      self.customized_class_names = nil
+
       add 'enumeration'
       self.form_partial = 'custom_fields/formats/enumeration'
 
diff --git a/test/unit/lib/redmine/field_format/enumeration_format_test.rb b/test/unit/lib/redmine/field_format/enumeration_format_test.rb
index 3903a3d50..15b1e3335 100644
--- a/test/unit/lib/redmine/field_format/enumeration_format_test.rb
+++ b/test/unit/lib/redmine/field_format/enumeration_format_test.rb
@@ -100,4 +100,8 @@ class Redmine::EnumerationFieldFormatTest < ActionView::TestCase
     assert_equal [@foo.id], @field.value_from_keyword('foo, baz', nil)
     assert_equal [], @field.value_from_keyword('baz', nil)
   end
+
+  def test_customized_class_names_should_overrided_to_nil
+    assert_nil Redmine::FieldFormat::EnumerationFormat.customized_class_names
+  end
 end