Optional List field in AWS Cloud Formation Template

Question

I'm trying to use an optional field in my CFT, where the field is actually a list of AZs in the account. by 'optional' I mean that the AZs selection should be take into account based on condition:

AWSTemplateFormatVersion: '2010-09-09'
Parameters:
  AvailabilityZones:
    Type: List<AWS::EC2::AvailabilityZone::Name>
  EnableAZLogic:
    Type: String
    Default: 'Disabled'
    AllowedValues:
      - 'Disabled'
      - 'Enabled'  
Conditions:   
  AZEnabled: !Equals [ !Ref EnableAZLogic, 'Enabled' ]
Resources:
  MyResource:
    Condition: AZEnabled
    Type: "AWS::CloudFormation::CustomResource"
    Properties:
      ServiceToken: !GetAtt MyResourceLambda.Arn
      StackName: !Ref "AWS::StackName"
      AvailabilityZones: !Ref AvailabilityZones
...   

but, when I select "Disabled" and select no AZs, I get this error:

Parameter validation failed: parameter value for parameter name AvailabilityZones does not exist

How to address that? I want the user to be able to select none of the AZs (the Lambda itself may check the list, if applicable)