Calling for_each flattened list, Invalid template interpolation value

Terraform Providers Azure
1

Hello, community! Trying to minimize my tf code and using flatten function to be called from module.

# root module
locals {
  key_vaults = [
    {
      kv_name = "my-super-key-vault-1"
      secrets = [
        {
          s_name  = "mysecret-key"
          s_value = "mysecret-value"
        }
      ]
    },
...
    {
      kv_name = "my-super-key-vault-2"
      secrets = []
    }
  ]

  group_policies = [
    {
      object_id          = "some_guid"
      key_permissions    = []
      secret_permissions = ["Get", "List"]
    }
...
  ]

# "Combining" local.key_vaults and local.group_policies objects properties
# into complex object:
  vaults_and_policies = distinct(flatten([
    for vault in local.key_vaults : [
      for team in local.group_policies : {
        kv_name            = vault.kv_name
        object_id          = team.object_id
        key_permissions    = team.key_permissions
        secret_permissions = team.secret_permissions
      }
    ]
  ]))
}

module "key_vaults" {
  source              = "./key_vaults"
  location            = data.azurerm_resource_group.my_rg.location
  resource_group_name = data.azurerm_resource_group.my_rg.name
  vaults_and_policies = local.vaults_and_policies
  tenant_id           = data.azurerm_subscription.current.tenant_id
}

# Output for troubleshotting:
output "vp" {
  value = local.vaults_and_policies
}

Child module:

variable "vaults_and_policies" {
  type = list(object({
    kv_name            = string
    object_id          = string
    key_permissions    = list(string)
    secret_permissions = list(string)
  }))
}
# the rest of variables
...

resource "azurerm_key_vault_access_policy" "this" {
  for_each = { for i in var.vaults_and_policies :
  "${i.kv_name}.${i.object_id}.${i.key_permissions}.${i.secret_permissions}" => i }

// referring to resourceIds. code ommitted for brevity.
  key_vault_id = azurerm_key_vault.this[each.value.kv_name].id
  object_id    = each.value.object_id
  tenant_id    = var.tenant_id

  certificate_permissions = each.value.certificate_permissions
  key_permissions         = each.value.key_permissions
  secret_permissions      = each.value.secret_permissions
  storage_permissions     = each.value.storage_permissions
}

It shows perfect thing I’d like to feed the module:

Changes to Outputs:
  + vp = [
      + {
          + key_permissions    = []
          + kv_name            = "my-super-key-vault-1"
          + object_id          = "some_guid"
          + secret_permissions = [
              + "Get",
              + "List",
            ]
        },
      + {
          + key_permissions    = []
          + kv_name            = "my-super-key-vault-2"
          + object_id          = "some_guid"
          + secret_permissions = [
              + "Get",
              + "List",
            ]
        },
    ]

terraform plan throws an error:

│ Error: Invalid template interpolation value
│
│   on key_vaults\kv.tf line 42, in resource "azurerm_key_vault_access_policy" "this":
│   42:   "${i.kv_name}.${i.object_id}.${i.key_permissions}.${i.secret_permissions}" => i... }
│     ├────────────────
│     │ i.secret_permissions is list of string with 2 elements
│
│ Cannot include the given value in a string template: string required.

│ Error: Invalid template interpolation value
│
│   on key_vaults\kv.tf line 42, in resource "azurerm_key_vault_access_policy" "this":
│   42:   "${i.kv_name}.${i.object_id}.${i.key_permissions}.${i.secret_permissions}" => i... }
│     ├────────────────
│     │ i.key_permissions is empty list of string
│
│ Cannot include the given value in a string template: string required.

What do I do wrong? Does for expects attributes of the same type?