LBSubnet error while EndpointType is API Gateway

As documentation suggests:

Subnets where the Application Load Balancer run. Leave it blank if you are using the API Gateway endpoint type.

We leave empty for API Gateway, then we get below error while creating stack:

Parameter validation failed: parameter value for parameter name LBSubnets does not exist. Rollback requested by user.

Start with the LBSubnets and EndpointType configuration described in the issue, then reproduce stack creation with API Gateway selected and LBSubnets empty. Trace why the LBSubnets parameter is still required. Done means the API Gateway configuration creates the stack without the validation error and the documentation matches the working behavior.

Written by the indexing model from the issue text.