Automation to transition status when assignee has been added

Hi, 

I would like it if we could automatically transition the status when an assignee has been set from Unassigned. 

For example, 
When the status is in Open, and I were to assign the issue to my team, the status will transition to work in progress. 

I have tried to trigger the automation when an Issue assigned into an
If block that checks if the Status = Open and the initiator is not Automation for Jira
Then it will transition from Open to Work in progress

The issue successfully transition to work in progress but the Assignee becomes "Automation for Jira". Is there any other way to do this where the Assignee will not be reassigned to "Automation for Jira"? 

Thanks for the help