Terraform command line arguments doesn't work with conditional expression

Question

I am trying to deploy a resource block either into dev or prod depending on the variable using conditional expression , for this i am trying to use command line argument. This work with terraform.tfvars but doesn't work with CMD arguments which means when i am trying to run terraform plan it doesn't not have any other changes.

ideally it should add 1 instance.

here is my resource block

main.tf file

resource "aws_instance" "dev" {

  ami           = "ami-0ca285d4c2cda3300"
  instance_type = var.instanceType
  count         = var.istest == true ? 1 : 0
}

resource "aws_instance" "prod" {

  ami           = "ami-0ca285d4c2cda3300"
  instance_type = "t2.micro"
  count         = var.istest == false ? 1 : 0
}

variables.tf

variable "istest" {
  default = true
}

terraform .tf vars is empty, command to run terraform

terraform plan -var="istest=false"