error when creating rds event subscription

Question

Below tf code is to create AWS RDS event subscriptions. Getting below error while running this piece of tf code. If source_ids are not passed then it seems to be using 'all' as the default value - https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/db_event_subscription

My requirement is not to not set it to 'all' in case event_subscription_source_type is not db-parameter-group. Is there any way to do that?

variable "event_subscription_source_type" {
  type        = map
  description = "Map of the event subscription and event source type"
  default = {
         "rds-events" = "db-instance"
         "db-param-group-chng" = "db-parameter-group"
      }
}

#creating rds event subscriptions
resource "aws_db_event_subscription" "event_subscriptions" {
  for_each         = toset(var.event_subscription_names)
  name             = each.value
  sns_topic        = var.sns_topic
  source_type      = var.event_subscription_source_type[each.value]
  source_ids       = var.event_subscription_source_type[each.value] != "db-parameter-group" ? null : "all"
...

Error

Error: Incorrect attribute value type
│   on base.tf line 58, in resource "aws_db_event_subscription" "event_subscriptions":
│   58:   source_ids       = var.event_subscription_source_type[each.value] != "db-parameter-group" ? null : "all"
│     │ each.value will be known only after apply
│     │ var.event_subscription_source_type is a map of dynamic, known only after apply
│ Inappropriate value for attribute "source_ids": set of string required.