I am creating an ALB via Cloudformation Template. One of the resources being created is a target group that will use EC2 instance ID’s that were exported during a previous job. I need to import the values and sub the import name to reflect different environment deployments. Running the cfn-lint tool, this is a correct template:

AWSTemplateFormatVersion: "2010-09-09"
Description: Creates an application load balancer.
Parameters:
  pSystem:
    Type: String
    Default: your-system-name
    Description: 'Name of the system. Default: your-system-name'
  pApp:
    Type: String
    Description: 'Name of the application solution'
  pEnvironment:
    Type: String
    Default: sbx
    Description: 'Name of the deployment tier. Default: sbx'
  pVpcId:
    Type: AWS::SSM::Parameter::Value<String>

Resources:

  rLoadBalancerTargetGroup:
    Type: AWS::ElasticLoadBalancingV2::TargetGroup
    Properties:
      Name: !Sub '${pSystem}-${pApp}-${pEnvironment}-public-tg'
      HealthCheckEnabled: true
      HealthCheckIntervalSeconds: 60
      HealthCheckPath: 'REDACTED'
      HealthCheckPort: 1234
      HealthCheckProtocol: HTTP
      HealthCheckTimeoutSeconds: 10
      HealthyThresholdCount: 2
      Matcher: 
        HttpCode: 200-299
      Port: 1234
      Protocol: HTTP
      ProtocolVersion: HTTP1
      TargetGroupAttributes: 
        - Key: 'deregistration_delay.timeout_seconds'
          Value: 300
        - Key: 'stickiness.enabled'
          Value: false
        - Key: 'load_balancing.algorithm.type'
          Value: round_robin
        - Key: 'slow_start.duration_seconds'
          Value: 0
      TargetType: instance
      UnhealthyThresholdCount: 2
      VpcId: !Ref pVpcId
      Targets: 
        - Id: !ImportValue 
          Fn::Sub: '${pSystem}-${pApp}-01-${pEnvironment}-ec2-id'
        - Id: !ImportValue
          Fn::Sub: "${pSystem}-${pApp}-02-${pEnvironment}-ec2-id"
        - Id: !ImportValue
          Fn::Sub: "${pSystem}-${pApp}-03-${pEnvironment}-ec2-id"

However, I receive this error in Cloudformation when it attempts to deploy:

No export named found

Which I have verified that all of my expected exports match this naming convention and are present in their respective stacks.

Reviewing the Template in Cloudformation, the template is translated to:

AWSTemplateFormatVersion: "2010-09-09"
Description: Creates an application load balancer.
Parameters:
  pSystem:
    Type: String
    Default: your-system-name
    Description: 'Name of the system. Default: your-system-name'
  pApp:
    Type: String
    Description: 'Name of the application solution'
  pEnvironment:
    Type: String
    Default: sbx
    Description: 'Name of the deployment tier. Default: sbx'
  pVpcId:
    Type: AWS::SSM::Parameter::Value<String>

Resources:

  rLoadBalancerTargetGroup:
    Type: AWS::ElasticLoadBalancingV2::TargetGroup
    Properties:
      Name:
        Fn::Sub: ${pSystem}-${pApp}-${pEnvironment}-public-tg
      HealthCheckEnabled: true
      HealthCheckIntervalSeconds: 60
      HealthCheckPath: /rest/web/system-healthy
      HealthCheckPort: 30600
      HealthCheckProtocol: HTTP
      HealthCheckTimeoutSeconds: 10
      HealthyThresholdCount: 2
      Matcher:
        HttpCode: 200-299
      Port: 30600
      Protocol: HTTP
      ProtocolVersion: HTTP1
      TargetGroupAttributes:
      - Key: deregistration_delay.timeout_seconds
        Value: 300
      - Key: stickiness.enabled
        Value: false
      - Key: load_balancing.algorithm.type
        Value: round_robin
      - Key: slow_start.duration_seconds
        Value: 0
      TargetType: instance
      UnhealthyThresholdCount: 2
      VpcId:
        Ref: pVpcId
      Targets:
      - Id:
          Fn::ImportValue: ''
        Fn::Sub: ${pSystem}-${pApp}-01-${pEnvironment}-ec2-id
      - Id:
          Fn::ImportValue: ''
        Fn::Sub: ${pSystem}-${pApp}-02-${pEnvironment}-ec2-id
      - Id:
          Fn::ImportValue: ''
        Fn::Sub: ${pSystem}-${pApp}-03-${pEnvironment}-ec2-id

Which has an empty string (”) after the Fn::ImportValue, and doesn’t appear to be reading the Sub command.

Any suggestions on how to import the EC2 instance ID’s for the target group?