Error: Invalid index - each.value is "foo" - module.x is object with 2 attributes

Terraform
1

i have the following terraform:

variable "some_list" {
  type = list(string)
  default = ["a", "b", "c", "d", "e"]
}

// this map has values that correspond to values in var.some_list, but only some of the
// entries in var.some_list have corresponding entries in this map
variable "some_map" {
  type = map(string)
  default = {
    "x": "a"
    "y": "c"
    "z": "e"
  }
}

// create an instance of some_module for each entry in var.some_list
// this module as a policy ARN as one of its outputs
module "some_module" {
  for_each    = toset(var.some_list)
}

// create an instance of an aws_iam_role for each key in some_ma
resource "aws_iam_role" "some_role" {
  for_each = var.some_map
  name = each.key
  assume_role_policy = data.aws_iam_policy_document.some_policy_doc[each.key].json
  tags = module.tags.tags
}

// attach a policy to the role selected based on the keys in var.some_map using the policyArn output from some_module based on the corresponding values in var.some_map 
resource "aws_iam_role_policy_attachment" "attachment" {
  for_each = var.some_map
  role = aws_iam_role.some_role[each.key].name
  policy_arn = module.some_module[each.value].policyArn
}

when i execute this, i get these errors:

Error: Invalid index
  
    on foo.tf line <x>, in resource "aws_iam_role_policy_attachment" "attachment":
    <x>:   policy_arn = module.some_module[each.value].policyArn
      ├────────────────
      │ each.value is "a"
      │ module.some_module is object with 2 attributes

Error: Invalid index
  
    on foo.tf line <x>, in resource "aws_iam_role_policy_attachment" "attachment":
    <x>:   policy_arn = module.some_module[each.value].policyArn
      ├────────────────
      │ each.value is "b"
      │ module.some_module is object with 2 attributes

and so on for “c”, “d” and “e”.

module.some_module should have exactly one instance for each value in var.some_list, so that lookup should work, shouldn’t it? what am i doing wrong?