Index: lib/redmine/field_format.rb
===================================================================
--- lib/redmine/field_format.rb	(revision 16211)
+++ lib/redmine/field_format.rb	(working copy)
@@ -793,6 +793,8 @@
             end
           end
           scope.sorted
+        elsif object.nil?
+          Principal.member_of(Project.visible.to_a).sorted.select {|p| p.is_a?(User)}
         else
           []
         end
Index: test/unit/custom_field_user_format_test.rb
===================================================================
--- test/unit/custom_field_user_format_test.rb	(revision 16211)
+++ test/unit/custom_field_user_format_test.rb	(working copy)
@@ -25,8 +25,10 @@
   end
 
   def test_possible_values_options_with_no_arguments
-    assert_equal [], @field.possible_values_options
-    assert_equal [], @field.possible_values_options(nil)
+    users = Principal.member_of(Project.visible.to_a).sorted.select {|p| p.is_a?(User)}
+    options = users.map {|u| [u.name, u.id.to_s]}
+    assert_equal options, @field.possible_values_options
+    assert_equal options, @field.possible_values_options(nil)
   end
 
   def test_possible_values_options_with_project_resource