diff --git a/app/models/custom_field.rb b/app/models/custom_field.rb
index 86227a4cf..22f29d3ac 100644
--- a/app/models/custom_field.rb
+++ b/app/models/custom_field.rb
@@ -53,6 +53,11 @@ class CustomField < ActiveRecord::Base
 
   CUSTOM_FIELDS_NAMES = CUSTOM_FIELDS_TABS.collect{|v| v[:name]}
 
+  def field_format=(arg)
+    # cannot change format of a saved custom field
+    super if new_record?
+  end
+
   def set_searchable
     # make sure these fields are not searchable
     self.searchable = false if %w(int float date bool).include?(field_format)
diff --git a/test/unit/custom_field_test.rb b/test/unit/custom_field_test.rb
index 547e712db..d858045ed 100644
--- a/test/unit/custom_field_test.rb
+++ b/test/unit/custom_field_test.rb
@@ -57,6 +57,12 @@ class CustomFieldTest < ActiveSupport::TestCase
     assert field.valid?
   end
 
+  def test_should_not_change_field_format_of_existing_custom_field
+    field = CustomField.find(1)
+    field.field_format = 'int'
+    assert_equal 'list', field.field_format
+  end
+
   def test_possible_values_should_accept_an_array
     field = CustomField.new
     field.possible_values = ["One value", ""]