Terraform Error: only lowercase alphanumeric characters and hyphens allowed in "cluster_identifier" rds_cluster

The detailed error is:

Error: only lowercase alphanumeric characters and hyphens allowed in "cluster_identifier"

  on ../rds_cluster/main.tf line 6, in resource "aws_rds_cluster" "mysql-cluster":
   6:   cluster_identifier = var.identifier

I get the above error while creating a rds_cluster from terraform. My main.tf file has the below code:

provider "aws"{
  region = var.region
}

resource "aws_rds_cluster" "mysql-cluster" {
  cluster_identifier = var.identifier
  availability_zones = var.azs
  database_name      = var.db_name
  master_username    = var.username
  master_password    = var.password
}

The variable.tf file is:

variable "region"{
 default = "us-east-1"
}

variable "identifier" {
  default = "aurora-cluster"
}

variable "db_name" {
  default = "mydb"
}

variable "username" {}

variable "password" {}

variable "azs" {}

And the module file that is calling this "rds_cluster" is:

provider "aws"{
  region = "us-east-1"
}

data "aws_availability_zones" "azs" {

}

data "aws_secretsmanager_secret_version" "creds" {
  secret_id = "db-creds"
}

module "mysql_aurora_instances" {
  source               =  "../rds_instance/"
  my_count             =  2
  identifier           =  "aurora-cluster"
  cluster_identifier   =  "mysql_aurora_cluster"
  instance_class       =  "db.t2.micro"
  engine               =  "aurora-mysql"
  engine_version       =  "5.7"
}

module "mysql_aurora_cluster" {
  source               =  "../rds_cluster/"
  identifier           =  "mysql_aurora_cluster" 
  azs                  =  data.aws_availability_zones.azs.names
  db_name              =  "my_db"
  username             =  "demo"
  password             =  data.aws_secretsmanager_secret_version.creds.secret_id
}

Points to note:

1. I am using terraform 0.14
2. When I perform terraform validate I get "Success! The configuration is valid."
3. However, when I perform terraform plan, I get the above error