Terraform "Invalid index" (list/map) in CI

By Kaveh Alemi·Latchkey

An expression indexes a list at a position that does not exist, or a map by a key it does not contain, so the lookup has no value to return.

What this error means

plan fails with "Invalid index", pointing at a [n] or ["key"] access. It surfaces when a list is empty/shorter than expected, or a map lookup uses a key that is not present.

Error: Invalid index

  on main.tf line 14, in resource "aws_subnet" "this":
  14:   availability_zone = var.azs[2]

The given key does not identify an element in this collection value: the given
index is greater than or equal to the length of the collection.

Common causes

List index out of range

Indexing var.azs[2] when the list has fewer than three elements has no element to return.

Map key missing

Looking up map["key"] where the key is absent fails; this is common with data that varies by environment.

How to fix it

Guard the access with length/lookup defaults

Use try() or lookup() with a default, or validate the collection length first.

availability_zone = try(var.azs[2], var.azs[length(var.azs) - 1])
# for maps:
name = lookup(var.tags, "Name", "default")

Validate input shapes

1. Add a variable validation that the list has enough elements.
2. Prefer for_each over count when iterating a map so keys are explicit.
3. Confirm environment-specific maps contain every key the config reads.

How to prevent it

• Guard list/map access with try() or lookup() defaults.
• Validate collection lengths and required keys in variable blocks.
• Prefer for_each over positional indexing where possible.

Frequently asked questions

Related guides

References

• Terraform documentation
• GitHub Actions documentation