Aws_lb_listener_rule - Weighted Target groups - Error

Hi,
I am trying to add 2 target groups to a single listener in AWS usign terraform 0.11.
Though the official terraform registry documentation(https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/lb_listener_rule) includes ‘Weighted target groups using forward block’, when I try to use that, am ending up with an error ‘aws_lb_listener_rule invalid or unknown key: forward’.
Could anyone please help me on this.