Redmine

When trying to use a plugin (WorkTime from r-labs.org/) I have an internal error only if I have a time entry custom field.
I post the bug here because the error occured in Redmine core and not in plugin code.
I also posted the bug on r-lab bugtraker : http://www.r-labs.org/issues/1385.

Started GET "/redmine/work_time/show/my-project-id?day=8&month=7&prj=false&user=7&year=2015" for 10.1.4.22 at 2015-07-08 09:17:40 +0200
Processing by WorkTimeController#show as HTML
  Parameters: {"day"=>"8", "month"=>"7", "prj"=>"false", "user"=>"7", "year"=>"2015", "id"=>"my-project-id"}
  Current user: myuserid (id=7)
  Rendered plugins/redmine_work_time/app/views/work_time/_select_project.html.erb (17.8ms)
  Rendered plugins/redmine_work_time/app/views/work_time/_select_user.html.erb (1.7ms)
  Rendered plugins/redmine_work_time/app/views/work_time/_user_month_table.html.erb (33.9ms)
  Rendered plugins/redmine_work_time/app/views/work_time/_user_day_table.html.erb (17.5ms)
  Rendered plugins/redmine_work_time/app/views/work_time/show.html.erb within layouts/base (73.9ms)
Completed 500 Internal Server Error in 143ms (ActiveRecord: 9.8ms)

ActionView::Template::Error (undefined method `value_was' for #<TimeEntryCustomField:0x0000000b448c60>):
    166:       nil
    167:     end
    168:   %>
    169:      <td><%= custom_field_tag "new_time_entry["+issue_id.to_s+"][0]", cf %></td>
    170:   <% end %>
    171: </tr>
    172: <%
  lib/redmine/field_format.rb:615:in `possible_custom_value_options'
  lib/redmine/field_format.rb:484:in `select_edit_tag'
  lib/redmine/field_format.rb:454:in `edit_tag'
  app/helpers/custom_fields_helper.rb:73:in `custom_field_tag'

It seems that a custom field of type TimeEntryCustomField does not have the value_was method.

My configuration :

Environment:
  Redmine version                3.0.3.stable
  Ruby version                   1.9.3-p448 (2013-06-27) [x86_64-linux]
  Rails version                  4.2.1
  Environment                    production
  Database adapter               Mysql2
SCM:
  Subversion                     1.6.17
  Git                            1.7.10.4
  Filesystem                     
Redmine plugins:
  redmine_base_deface            0.0.1
  redmine_custom_workflows       0.1.0
  redmine_latex_mathjax          0.1.0
  redmine_maintenance_mode       2.0.1
  redmine_work_time              0.3.0
  redmine_workflow_enhancements  0.4.0