Error: Incorrect attribute value type

Terraform Providers AWS
1

Hello, not sure if this is correct forum to place issues in but I am running into an issue when trying to add users to specific group. The group is successfully created, the users are successfully created, but when trying to add users to group it does not work. Is there something that I am doing wrong is is this a bug? I get the following error:

Terraform v0.12.24

Error

Error: Incorrect attribute value type  

on: main.tf line 46, in resource "aws_iam_group_membership" "team":
46:   users = each.key  

Inappropriate value for attribute "users": set of string required.

terraform.tfvars

user_names = {
  "dummy1"   = {
    path          = "/"
    force_destroy = true
    tag_fullname  = "dummy 1"
  }
  "dummy2"   = {
    path          = "/"
    force_destroy = true
    tag_fullname  = "dummy 2"
  }
}

group_memberships = {
  "dummy1" = ["dummy-group"]
  "dummy2" = ["dummy-group"]
}  

group  = "dummy-group"

modules main.tf

### --------------------------------------
### Create user account
### --------------------------------------

resource "aws_iam_user" "username" {
  for_each = var.user_names
  name          = each.key
  path          = each.value["path"]
  force_destroy = each.value["force_destroy"]

  # Full Name is tag Key, and each.value["tag_fullname"] is tag value
  tags = map("Full Name", each.value["tag_fullname"])
}


### --------------------------------------
### Add user to group
### --------------------------------------
resource "aws_iam_group_membership" "team" {
  name = var.group

  for_each = var.group_memberships
  users = each.key
  group = each.value
  depends_on = [aws_iam_user.username]
}