Cannot Mount CD-ROM to a powered on VM

I used 'Mount CD-ROM' workflow to mount an iso file to a powered on VM. But it returned error as following.

Found IDE controller (Key: 200) for busNumber 0

[2015-10-30 16:07:13.226] [I] Task 'ReconfigVM_Task' error: The attempted operation cannot be performed in the current state (Powered on). (Dynamic Script Module name : vim3WaitTaskEnd#20)

Am I missing something?