Terraform lookup - Inappropriate value for attribute "cidr_blocks": list of string required

Terraform
1

Hello,

I’m trying to create a modular Security group rule by using lookup on the following variable:

ingress_rules       = [
        {
            description = "HTTP from anywhere"
            from_port   = "80"
            to_port     = "80"
            protocol    = "tcp"
            **cidr_blocks = ["0.0.0.0/0"]**
        },
        {
            description = "HTTPS from anywhere"
            from_port   = "443"
            to_port     = "443"
            protocol    = "tcp"
            **cidr_blocks = ["0.0.0.0/0"]**
        }
    ]

This is the resource where I’m performing the lookup:

resource "aws_security_group_rule" "ingress_rules" {
  count = length(var.ingress_rules)

  security_group_id = aws_security_group.security_group
  type              = "ingress"

  **cidr_blocks       = lookup(var.ingress_rules[count.index], "cidr_blocks", null)**
  description       = lookup(var.ingress_rules[count.index], "description", null)

  from_port         = lookup(var.ingress_rules[count.index], "from_port", null)
  to_port           = lookup(var.ingress_rules[count.index], "to_port", null)
  protocol          = lookup(var.ingress_rules[count.index], "protocol", null)
}

The error I receive is the following:

Error: Incorrect attribute value type
│
│   on main.tf line 38, in resource "aws_security_group_rule" "ingress_rules":
│   38:   cidr_blocks       = lookup(var.ingress_rules[count.index], "cidr_blocks", null)
│     ├────────────────
│     │ count.index is a number, known only after apply
│     │ var.ingress_rules is a list of map of string, known only after apply
│
│ Inappropriate value for attribute "cidr_blocks": list of string required.

Is this a limitation of the lookup function ? Can you please let me know if there is a workaround for this ?