Can't remove EC2 instance from Placement Group, Maybe bug?

I followed instructions from here
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/placement-groups.html#change-instance-placement-group
And used command

aws ec2 modify-instance-placement --instance-id i-0123a456700123456 --group-name ""

But I'm getting error

error: argument --group-name: expected one argument

Also would be good idea to set "if else" or whatever to prevent users to add "t" instances to placement group.
I can't start instance, and command not works, the only way is to export it to AMI and run like new instance? :)