title: AWS Cloudformation Value of property SubnetIds must be of type List of String
body: <p>I am tying this cloudformation stack but when creation reaches DBSubnetGroup it fails with the message: Value of property SubnetIds must be of type List of String. I appreciate your help.
  All seems working fine except the DBSubnetGroup.</p>
  <pre><code>AWSTemplateFormatVersion: 2010-09-09
  Description: My First CloudFormation Template
  Resources:
    MyVPC:
      Type: AWS::EC2::VPC
      Properties:
        CidrBlock: 10.1.0.0/16
        EnableDnsHostnames: true
        EnableDnsSupport: true
        InstanceTenancy: default
        Tags:
        - Key: Name
          Value: VPC for Activity Number 1 
    MySubnet:
      Type: AWS::EC2::Subnet
      Properties:
        VpcId: !Ref MyVPC
        CidrBlock: 10.1.1.0/24
        MapPublicIpOnLaunch: true
        AvailabilityZone: &quot;us-east-1c&quot;
        Tags:
        - Key: Name
          Value: Public Subnet 
    MySubnetA:
      Type: AWS::EC2::Subnet
      Properties:
        VpcId: !Ref MyVPC
        CidrBlock: 10.1.2.0/24
        MapPublicIpOnLaunch: true
        AvailabilityZone: &quot;us-east-1a&quot;
        Tags:
        - Key: Name
          Value: Public Subnet  A
    MySubnetB:
      Type: AWS::EC2::Subnet
      Properties:
        VpcId: !Ref MyVPC
        CidrBlock: 10.1.3.0/24
        MapPublicIpOnLaunch: true
        AvailabilityZone: &quot;us-east-1b&quot;
        Tags:
        - Key: Name
          Value: Public Subnet B      
  </code></pre>
  <p>That's where creation failure starts:</p>
  <pre><code>  DBSubnetGroup:
      Type: AWS::RDS::DBSubnetGroup
      DependsOn:
        - MyVPC
        - MySubnet
        - MySubnetA
        - MySubnetB
      Properties:
        DBSubnetGroupDescription: Subnets to lauch the database
        SubnetIds: 
          -!Ref MySubnet
          -!Ref MySubnetA  
          -!Ref MySubnetB    
          
        
  
  </code></pre>