ActionView::Template::Error (undefined method `-' for nil:NilClass after data migration from Mantis

Environment:
Redmine version 2.6.0.stable
Ruby version 1.9.3-p484 (2013-11-22) [x86_64-linux]
Rails version 3.2.19
Environment production
Database adapter Mysql2
SCM:
Subversion 1.8.8
Git 1.9.1
Filesystem
Redmine plugins:
redmine_contacts 3.4.4
redmine_contacts_invoices 3.2.3

I did a clean install of Redmine using this guide: http://www.redmine.org/projects/redmine/wiki/HowTo_Install_Redmine_25x_on_Ubuntu_1404_with_Apache2_Phusion_Passenger_MySQL_and_Subversion

Then I migrated my Mantis Database using this guide: http://foaa.de/old-blog/2010/04/non-destructive-migration-from-mantis-to-redmine/

I get the error when trying to view issues migrated from Mantis, all other functions work great. This is my first Redmine install.

Thanks for your help,

John

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ActionView::Template::Error (undefined method `-' for nil:NilClass after data migration from Mantis