Nested Fn::ImportValue in Fn::Sub not working for SAM template - aws-api-gateway

Question

Description:
I am trying to define Serverless API resource. But having trouble in defining location of swagger specification file using function ImportValue.
Steps to reproduce the issue:
I am not able to define AWS::Serverless::Api resource having nested function ImportValue in Location. I have tried following three ways, none of them work.
Note: Stack parameters are defined properly and export value from other stack exists. Not showing them here for brevity reason.
ApiGatewayApi:
Type: AWS::Serverless::Api
Properties:
Name: !Sub ${AWS::StackName}-API
StageName: !Ref ApiGatewayStageName
DefinitionBody:
'Fn::Transform':
Name: 'AWS::Include'
Parameters:
Location:
Fn::Sub:
- s3://${BucketName}/${SwaggerSpecificationS3Key}
- BucketName:
Fn::ImportValue:
!Sub "${EnvironmentName}-dist-bucket-${AWS::Region}"
ApiGatewayApi:
Type: AWS::Serverless::Api
Properties:
Name: !Sub ${AWS::StackName}-API
StageName: !Ref ApiGatewayStageName
DefinitionBody:
'Fn::Transform':
Name: 'AWS::Include'
Parameters:
Location:
Fn::Sub:
- s3://${BucketName}/${SwaggerSpecificationS3Key}
- BucketName:
!ImportValue 'dev-dist-bucket-us-east-1'
ApiGatewayApi:
Type: AWS::Serverless::Api
Properties:
Name: !Sub ${AWS::StackName}-API
StageName: !Ref ApiGatewayStageName
DefinitionBody:
'Fn::Transform':
Name: 'AWS::Include'
Parameters:
Location:
Fn::Sub:
- s3://${BucketName}/${SwaggerSpecificationS3Key}
- BucketName:
Fn::ImportValue: 'dev-dist-bucket-us-east-1'
Cloudformation shows following error.
FAILED - The value of parameter Location under transform Include must
resolve to a string, number, boolean or a list of any of these.
However, if I do not use ImportValue it works with a nested Fn::Sub
ApiGatewayApi:
Type: AWS::Serverless::Api
Properties:
Name: !Sub ${AWS::StackName}-API
StageName: !Ref ApiGatewayStageName
DefinitionBody:
'Fn::Transform':
Name: 'AWS::Include'
Parameters:
Location:
Fn::Sub:
- s3://${BucketName}/${SwaggerSpecificationS3Key}
- BucketName:
Fn::Sub: dist-bucket-${EnvironmentName}-${AWS::Region}
Is it because of Fn::Transform or AWS::Include?