Setting up a new Jenkins server and adding the Jira configuration info in the main Admin Configuration settings.  There is a place to add the Thread Executor Size but it will not accept any value nor blank. Regardless of value put in you cannot Validate and get the error that the Thread Executor Size must be at least 1. 

I've tried setting it 0, 1, 9, 10, 12 and also blank but always get the same error.

You can still save changes but cannot validate.