How to fix Error: Invalid index in Terraform

TerraformINTERMEDIATEMEDIUM

The 'Invalid index' error occurs when Terraform tries to access an element in a list, tuple, or map using an index or key that doesn't exist. This commonly happens with empty collections, out-of-bounds array access, or missing map keys. The fix involves checking collection bounds, using conditional expressions, or applying the try() function.

What this error means

Terraform's configuration language allows you to access elements in lists, tuples, and maps using bracket notation (e.g., var.subnets[0] or var.tags["key"]). When Terraform evaluates your configuration, it validates that the index or key you're referencing actually exists in the collection. If it doesn't—either because the collection is empty, the index is too large, or the key doesn't exist—Terraform raises an 'Invalid index' error. This prevents your configuration from being parsed and applied.

How to fix "Error: Invalid index"

1Identify the line causing the error

Review the full error message from terraform plan or apply. The error will show:

bash

Error: Invalid index

  on main.tf line X, in resource "aws_instance" "example":
   X:   availability_zone = var.availability_zones[0]

The given key does not identify an element in this collection value: the collection has no elements.

Note the exact file, line, and variable/reference being accessed (in this example: var.availability_zones[0]).

2Check if the collection is empty

Verify whether the collection you're accessing has any elements. If you're referencing a variable, check its default value or how it's being set:

hcl

variable "availability_zones" {
  type    = list(string)
  default = []  # This is empty!
}

# This will fail:
resource "aws_instance" "example" {
  availability_zone = var.availability_zones[0]
}

If the list is intentionally empty or may be empty, you need to handle that case.

3Use the try() function for safe access

The try() function attempts to evaluate an expression and returns a default value if it fails. This is the simplest fix for potential invalid index errors:

hcl

# Instead of:
availability_zone = var.availability_zones[0]

# Use:
availability_zone = try(var.availability_zones[0], "us-east-1a")

Or return null if no default is appropriate:

hcl

availability_zone = try(var.availability_zones[0], null)

The try() function will evaluate the expression safely and return the fallback value if the index is invalid.

4Use length() to check bounds before accessing

For more explicit control, use the length() function to verify the collection has elements before accessing by index:

hcl

resource "aws_instance" "example" {
  # Only set if the list has at least one element
  availability_zone = length(var.availability_zones) > 0 ? var.availability_zones[0] : "us-east-1a"
}

Or use it in a conditional to skip the resource entirely:

hcl

resource "aws_instance" "example" {
  count             = length(var.availability_zones) > 0 ? 1 : 0
  availability_zone = var.availability_zones[0]
}

5For maps: use lookup() or the get alternative

When accessing map keys that might not exist, use lookup() to provide a default:

hcl

# Instead of:
environment = var.tags["environment"]

# Use:
environment = lookup(var.tags, "environment", "development")

Or use the conditional operator with contains() to check first:

hcl

environment = contains(keys(var.tags), "environment") ? var.tags["environment"] : "development"

6Fix count or for_each references

If using count or for_each, ensure your reference matches the iteration mode:

hcl

# With count:
resource "aws_instance" "example" {
  count = var.instance_count
  # ...
}

# Later, reference by index (0-based):
resource "aws_security_group_rule" "example" {
  security_group_id = aws_instance.example[0].security_group_id  # Correct
}

# With for_each:
resource "aws_instance" "example" {
  for_each = var.instances  # Map or set
  # ...
}

# Reference by key:
resource "aws_security_group_rule" "example" {
  security_group_id = aws_instance.example["web"].security_group_id  # Correct
}

Ensure the key or index you're using actually exists in the iteration.

7Validate your variable inputs

Add validation to your variables to prevent empty collections from being passed in:

hcl

variable "availability_zones" {
  type = list(string)
  
  validation {
    condition     = length(var.availability_zones) > 0
    error_message = "availability_zones must not be empty."
  }
}

This catches the problem early with a clear error message before Terraform tries to evaluate your resources.

8Test with terraform plan

After making changes, run a plan to verify the error is resolved:

bash

terraform plan

If you see a different error or the same error on a different line, repeat steps 1-3 for the next invalid reference. Once plan succeeds, you can proceed with:

bash

terraform apply

How to fix Error: Invalid index in Terraform

What this error means

Typical symptoms

Common causes

How to fix "Error: Invalid index"

Advanced notes

Related errors

Official resources & further reading