Error: Error in function call

  on prometheus.tf line 20, in resource "aws_instance" "prometheus":
  20:   subnet_id                   = "${element(data.aws_subnet_ids.private.ids, count.index)}" # subnet-private-1c
    |----------------
    | count.index is 0
    | data.aws_subnet_ids.private.ids is set of string with 3 elements

Call to function "element" failed: cannot read elements from set of string.

ws_subnet_ids. = [
  [
    "subnet-0229ce875150ccf38",
    "subnet-0429e7fc67e8651b0",
    "subnet-0b593740c2e8322a4",
  ],
]

data "aws_subnet_ids" "private" {
    vpc_id = "vpc-05ecf2b49733444fb"

    filter {
      name   = "tag:Name"
      values = ["subnet-private-1a", "subnet-private-1b", "subnet-private-1c"]
    }
  }

  count                       = 1
#  subnet_id                   = "subnet-0b593740c2e8322a4" # subnet-private-1c
  subnet_id                   = "${data.aws_subnet_ids.private.ids, count.index)}" # subnet-private-1c

ws_subnet_ids. = [
  [
    "subnet-0229ce875150ccf38",
    "subnet-0429e7fc67e8651b0",
    "subnet-0b593740c2e8322a4",
  ],
]