ApiGatewayV2::Authorizer - "Internal Failure"

Hi, I'm trying to create an API Gateway authorizer via CloudFormation, and am getting "Internal Failure" when adding the API Autorizer shown below on deploying. Here's the segment:

  ApiAuthorizer:
    Type: AWS::ApiGatewayV2::Authorizer
    Properties:
      Name: MyCustomAuthorizer
      # "Api" is my CloudFormation API which gets created ok...
      ApiId: $Ref Api
      AuthorizerType: REQUEST
      # AuthorizerFunctionARN is a parameter set to the Lambda function's ARN
      AuthorizerUri:
        'Fn::Sub': >-
          arn:aws:apigateway:${AWS::Region}:lambda:path/2015-03-31/functions/${AuthorizerFunctionARN}/invocation      
      # I've tried with and without AuthorizerCredentialsArn
      AuthorizerCredentialsArn: !Sub "arn:aws:iam::${AWS::AccountId}:role/APIGatewayLambdaInvokeRole"
      IdentitySource:
        - route.request.header.Auth

  ApiAuthorizerPermission:
    Type: AWS::Lambda::Permission
    Properties:
      Action: lambda:InvokeFunction
      FunctionName: !Ref AuthorizerFunctionARN
      Principal: apigateway.amazonaws.com
      SourceArn: !Sub "arn:aws:execute-api:${AWS::Region}:${AWS::AccountId}:${Api}/authorizers/${ApiAuthorizer}" 

I found this on GitHub and this on StackOverflow but not making any headway. Is there a way to get more detailed error info from CloudFormation than "Internal Failure"? Is there some permissions I need to set up? TIA