raise ValueError('Name "%s" not alphanumeric' % self.title)

Hi,

I am trying to create an empty DB Cluster. While running "pipenv run invoke compile" I get "raise ValueError('Name "%s" not alphanumeric' % self.title)" for MasterUsername / MasterUserPassword.

I am trying to get the value from CCM parameter store but I get this error on the line where I am declaring it as a Parameter. Can someone please help.

---code for the same---

PG_Username = Parameter(
"PG_Username",
Description="PG User Name for RDS",
Type="String",
Default="/dev/unixyz/ordering/rds/PGUsername",
)
template.add_parameter(PG_Username)

## Mappings
template.add_mapping(
"MstrUsername",
{
"dev": {
"ARN": Sub("arn:aws:ssm:eu-west-1:1234567890:parameter${PG_Username}")
},
}
)

## Calling it in DB Cluster definition
MasterUsername=FindInMap("MstrUsername", Ref(EnvironmentName), "ARN"),