Diff on Merge Requests not showing up

I see the following error in the log file. Any help with resolving this issue is much appreciated

==> /var/log/gitlab/gitlab-rails/production.log <==
Started GET "/<group>/<project_name>/merge_requests/131/diffs.json" for <ip_address> at 2016-10-13 05:43:13 +0000
Processing by Projects::MergeRequestsController#diffs as JSON
  Parameters: {"namespace_id"=>"<group>", "project_id"=>"<project_name>", "id"=>"131"}
Started GET "/<group>/dallas/merge_requests/249/ci_status" for <ip_address> at 2016-10-13 05:43:13 +0000
Processing by Projects::MergeRequestsController#ci_status as JSON
  Parameters: {"namespace_id"=>"<group>", "project_id"=>"dallas", "id"=>"249"}
Completed 500 Internal Server Error in 72ms (ActiveRecord: 4.9ms)

ActionView::Template::Error (undefined method `[]' for nil:NilClass):
    25:                         latest version
    26:                       - else
    27:                         version #{version_index(merge_request_diff)}
    28:                     .monospace #{short_sha(merge_request_diff.head_commit_sha)}
    29:                     %small
    30:                       #{number_with_delimiter(merge_request_diff.commits.count)} #{'commit'.pluralize(merge_request_diff.commits.count)},
    31:                       = time_ago_with_tooltip(merge_request_diff.created_at)
  app/models/commit.rb:42:in `truncate_sha'
  app/helpers/git_helper.rb:7:in `short_sha'
  app/views/projects/merge_requests/show/_versions.html.haml:28:in `block (2 levels) in _app_views_projects_merge_requests_show__versions_html_haml___591203716679907293_80875340'
  app/views/projects/merge_requests/show/_versions.html.haml:22:in `block in _app_views_projects_merge_requests_show__versions_html_haml___591203716679907293_80875340'
  app/views/projects/merge_requests/show/_versions.html.haml:20:in `_app_views_projects_merge_requests_show__versions_html_haml___591203716679907293_80875340'
  app/views/projects/merge_requests/show/_diffs.html.haml:2:in `_app_views_projects_merge_requests_show__diffs_html_haml___4492578656062802115_81458900'
  app/controllers/application_controller.rb:199:in `view_to_html_string'
  app/controllers/projects/merge_requests_controller.rb:104:in `block (2 levels) in diffs'
  app/controllers/projects/merge_requests_controller.rb:95:in `diffs'
  lib/gitlab/request_profiler/middleware.rb:15:in `call'
  lib/gitlab/middleware/go.rb:16:in `call'

Environment Details:

• Gitlab CE Omnibus
• version 8.12.6