Providing master_ipv4_cidr_block and subnet range using terraform variables in GCP

I have created a private cluster in GCP using terraform modules. As per the documentation here: https://www.terraform.io/docs/providers/google/r/container_cluster.html#master_ipv4_cidr_block, I have configured the private cluster as below:

 private_cluster_config { 
   enable_private_endpoint = true   
   enable_private_nodes    = true 
   master_ipv4_cidr_block  = "${cidrsubnet(var.cidr, 28, 1)}"
      } 

This cluster is provisioned in a subnet whose CIDR range is 10.15.0.0/16 (var.cidr is set to 10.15.0.0/16).

When I run terraform apply, I get the below error:

Error waiting for creating GKE cluster: The given master_ipv4_cidr 10.15.0.16/28 overlaps with an existing network 10.15.0.0/16.
"${cidrsubnet(var.cidr, 12, 1)}"

How do I provide the master_ipv4_cidr_block IPV4 address range and subnet range using value provided in var.cidr so that the ranges dont overlap? How should the cidrsubent() be modified to suit this requirement?