diff --git a/app/views/custom_fields/formats/_list.html.erb b/app/views/custom_fields/formats/_list.html.erb
index 675bbbf8d..f615e5057 100644
--- a/app/views/custom_fields/formats/_list.html.erb
+++ b/app/views/custom_fields/formats/_list.html.erb
@@ -1,5 +1,5 @@
 <p>
-  <%= f.text_area :possible_values, :value => @custom_field.possible_values.to_a.join("\n"), :rows => 15 %>
+  <%= f.text_area :possible_values, :value => @custom_field.possible_values.to_a.join("\n"), :rows => 15, :required => true %>
   <em class="info"><%= l(:text_custom_field_possible_values_info) %></em>
 </p>
 <p><%= f.text_field(:default_value) %></p>