AWS add option group

I signed up for paid support and here's the answer, in case it helps anyone else. I had to edit the trust policy on the IAM role like so:

{
    "Version": "2012-10-17",
    "Statement":
    [{
        "Effect": "Allow",
        "Principal": {"Service":  "rds.amazonaws.com"},
        "Action": "sts:AssumeRole"
    }]
}

Then I was able to add the option to the option group and the restore worked. Note that the trust policy is called "trust relationship" on the web console.