Create DBSubnetGroup SubnetIds from ASW::EC2::Subnet:Id in CloudFormation

I am trying to create an Aurora MySQL RDS using CloudFormation. The code below is just a snippet of what it is doing. The issue I am having is that the SubnetIds for the DBSubnetGroup is not accepting what is being passed. It either errors out with receiving jsonarray instead of string. Or receiving jsonobject instead of jsonarray. Has anyone worked on this?

AWSTemplateFormatVersion: "2010-09-09"
Metadata:
  AWS::CloudFormation::Interface: 
      ParameterGroups: 
      - 
        Label: 
          default: "Network Configuration"
        Parameters:
          - MainRegion
          - ClusterPrefix
          - VPCName
          - VPCSecurityGroupIds
          - SelectedSubnetIds

Parameters:

  MainRegion:
    Type: String
    Default: us-west-2
    AllowedValues:
      - us-east-1
      - us-east-2
      - us-west-1
      - us-west-2      
    Description: Select the main region of choice.

  ClusterPrefix:
    Description: Enter (in lowercase) the cluster prefix that will be added to the begining of the RDS resource
    Type: String
    Default: abc

  VPCName:
    Description: Select the VPC for the RDS Aurora MySQL
    Type: AWS::EC2::VPC::Id

  VPCSecurityGroupIds:
    Description: Select the VPC Security Group
    Type: "List<AWS::EC2::SecurityGroup::Id>"

  SelectedSubnetIds:
    Description: Select the subnets
    Type: "List<AWS::EC2::Subnet::Id>"


Resources:

    RDSDBSubnetGroup:
        Type: "AWS::RDS::DBSubnetGroup"
        Properties:
            DBSubnetGroupDescription: "Subnet Group for Name Global DB"
            DBSubnetGroupName: "${ClusterPrefix}-name-db-sg"
            SubnetIds: 
               - !Ref SelectedSubnetIds
              #[ SelectedSubnetIds ]
              #'!ForEach::SelectValue':
              #  - SelectedSubnet
              #  - !Ref SelectedSubnetIds
                #- SelectedSubnet
                #- !Ref 'Fn::Sub': '${SelectedSubnet}'
                #- !Join [ ",", [ SelectedSubnet ] ]
               # - Fn::ToJsonString SelectedSubnet
                #- ListOutput: true