title: Value of property SecurityGroupIds must be of type List of String error while updating stack
body: <p>I am getting ROLLBACK_COMPLETE while try to updating a stack using the following code. Under events, I am not getting an error as "Value of property SecurityGroupIds must be of type List of String".please help me to find a solution.</p>
  
  <p>Mycode for first stack:</p>
  
  <pre><code>Resources:
    myvpc:
      Type: AWS::EC2::VPC
      Properties:
          CidrBlock: 10.0.0.0/16
          EnableDnsSupport: true
          EnableDnsHostnames: true
          InstanceTenancy: default
          Tags:
              - Key: Name
                Value: myvpc
  
   myinternetgateway:
      Type: AWS::EC2::InternetGateway
      Properties:
          Tags: 
              - Key: Name
                Value: mygtwy
  
   mygatewayattach:
      Type: AWS::EC2::VPCGatewayAttachment
      Properties:
          InternetGatewayId: !Ref myinternetgateway
          VpcId: !Ref myvpc
  
   mysubnet1:
      Type: AWS::EC2::Subnet
      Properties:
          AvailabilityZone: us-east-1a
          VpcId: !Ref myvpc
          CidrBlock: 10.0.1.0/24
          MapPublicIpOnLaunch: true
  
   Routetable:
      Type: AWS::EC2::RouteTable
      Properties:
          VpcId: !Ref myvpc
  
   Route:
      Type: AWS::EC2::Route
      DependsOn: myinternetgateway
      Properties:
          DestinationCidrBlock: 0.0.0.0/0
          GatewayId: !Ref myinternetgateway
          RouteTableId: !Ref Routetable
  
   SubnetARouteTableAssociation:
      Type: AWS::EC2::SubnetRouteTableAssociation
      Properties:
          RouteTableId: !Ref Routetable
          SubnetId: !Ref mysubnet1
  </code></pre>
  
  <p>On update, I added the following. During this time I am getting the error I mentioned earlier</p>
  
  <pre><code> Myec2:
      Type: 'AWS::EC2::Instance'
      Properties:
          SecurityGroupIds:
              - !Ref Mysecgroup
          KeyName: !Ref KeyName
          ImageId: ami-0922553b7b0369273
          InstanceType: t2.micro
          SubnetId: !Ref mysubnet1
  
   Mysecgroup:
      Type: 'AWS::EC2::SecurityGroup'
      Properties:
          GroupDescription: Enable SSH access via port 22
          VpcId: !Ref myvpc
          SecurityGroupIngress:
              - IpProtocol: tcp
                FromPort: '22'
                ToPort: '22'
                CidrIp: 0.0.0.0/0  
  </code></pre>