Cloudformation issue with target groups and ASG

Question

I am trying to use cloudformation to setup a ASG and I have a target group which I would like to associate with the ASG. The target group is created okay but when I go to attach it to the ASG I get an error message. I'd like to be able to attach this as I want a ALB in front of the ASG

Error message:

Value of property TargetGroupARNs must be of type List of String

Cloudformation code

TargetGroup:
    Type: AWS::ElasticLoadBalancingV2::TargetGroup
    Properties:
      VpcId: 
        Fn::ImportValue: 
          !Sub "${Prefix}-VpcId"
      TargetType: instance
      Port: 443
      Protocol: HTTPS
AutoScalingGroup:
    Type: AWS::AutoScaling::AutoScalingGroup
    Properties:
      AutoScalingGroupName: Autoscaling Group Alpha
      Cooldown: 120
      DesiredCapacity: 1
      LaunchConfigurationName: AlphaLaunchConfiguration
      MaxSize: 1
      MinSize: 1
      Tags:
        - 
          Key: Name
          Value: Alpha ASG
          PropagateAtLaunch: 'true'
      TargetGroupARNs:
        Ref: TargetGroup

Answer 1

Resources:                                                                                                                                                                                  
    TargetGroup:                                                                                                                                                                              
        Type: AWS::ElasticLoadBalancingV2::TargetGroup                                                                                                                                        
        Properties:                                                                                                                                                                           
          VpcId:                                                                                                                                                                              
            Fn::ImportValue:                                                                                                                                                                  
              !Sub "${Prefix}-VpcId"                                                                                                                                                          
          TargetType: instance                                                                                                                                                                
          Port: 443                                                                                                                                                                           
          Protocol: HTTPS                                                                                                                                                                     
    AutoScalingGroup:                                                                                                                                                                         
        Type: AWS::AutoScaling::AutoScalingGroup                                                                                                                                              
        Properties:                                                                                                                                                                           
          AutoScalingGroupName: Autoscaling Group Alpha                                                                                                                                       
          Cooldown: "120"                                                                                                                                                                     
          DesiredCapacity: "1"                                                                                                                                                                
          LaunchConfigurationName: AlphaLaunchConfiguration                                                                                                                                   
          MaxSize: "1"                                                                                                                                                                        
          MinSize: "1"                                                                                                                                                                        
          Tags:                                                                                                                                                                               
            -                                                                                                                                                                                 
              Key: Name                                                                                                                                                                       
              Value: Alpha ASG                                                                                                                                                                
              PropagateAtLaunch: true                                                                                                                                                         
          TargetGroupARNs:                                                                                                                                                                    
            - Ref: TargetGroup

As the error message states, TargetGroupARNs value should be of type List. https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-as-group.html#cfn-as-group-targetgrouparns

Answer 2

Changing the value of "TargetGroupARNs:" from "Ref: TargetGroup" to "- !Ref TargetGroup" should solve the problem.

Reference :https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-as-group.html