Redmine

After attempting to upgrade using the 1.2-stable release in branches, I'm receiving the error below when selecting the "Issues" menu item for a project. I'm assuming this is data or configuration related because it does not occur with all of my projects. An action is being performed on a null value, but not sure which value and why it is null. Not sure if this is related, but projects generating this error have issue references to locked users.

Any help would be appreciated.

ActionView::TemplateError (undefined method `-' for nil:NilClass) on line #28 of app/views/issues/_list.rhtml:
25:             </tr>
26:             <% previous_group = group %>
27:   <% end %>
28:     <tr id="issue-<%= issue.id %>" class="hascontextmenu <%= cycle('odd', 'even') %> <%= issue.css_classes %> <%= level > 0 ? "idnt idnt-#{level}" : nil %>">
29:       <td class="checkbox hide-when-print"><%= check_box_tag("ids[]", issue.id, false, :id => nil) %></td>
30:             <td class="id"><%= link_to issue.id, :controller => 'issues', :action => 'show', :id => issue %></td>
31:         <% query.columns.each do |column| %><%= content_tag 'td', column_content(column, issue), :class => column.css_classes %><% end %>

    app/models/issue.rb:571:in `css_classes'
    app/views/issues/_list.rhtml:28
    app/helpers/issues_helper.rb:27:in `issue_list'
    app/helpers/issues_helper.rb:23:in `each'
    app/helpers/issues_helper.rb:23:in `issue_list'
    app/views/issues/_list.rhtml:16
    app/views/issues/_list.rhtml:1:in `_run_rhtml_app47views47issues47_list46rhtml_locals_issues_list_object_query'
    app/views/issues/index.rhtml:51:in `_run_rhtml_app47views47issues47index46rhtml'
    app/controllers/issues_controller.rb:91
    app/controllers/issues_controller.rb:90:in `index'
    /usr/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
    /usr/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
    /usr/lib/ruby/1.8/webrick/server.rb:173:in `start_thread'
    /usr/lib/ruby/1.8/webrick/server.rb:162:in `start'
    /usr/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
    /usr/lib/ruby/1.8/webrick/server.rb:95:in `start'
    /usr/lib/ruby/1.8/webrick/server.rb:92:in `each'
    /usr/lib/ruby/1.8/webrick/server.rb:92:in `start'
    /usr/lib/ruby/1.8/webrick/server.rb:23:in `start'
    /usr/lib/ruby/1.8/webrick/server.rb:82:in `start'

Rendering /opt/redmine-1.2/public/500.html (500 Internal Server Error)