When customising a Nintex form for a document set content type the error below is displayed below the ribbon of Nintex forms designer

Error - Cannot use Nintex for content type: Document Set

This was identified as a bug in Nintex Forms 2013 version 2.11.1.10

To resolve the issue, please upgrade to the latest version of Nintex Form 2013

This issue was identified on Nintex Form 2013 version 2.11.1.10 as a defect and has been fixed on version 2.11.2.0