Terraform "Missing required argument" for a module variable in CI

By Kaveh Alemi·Latchkey

A module declares an input variable with no default, so it is required. The module block that calls it does not pass a value for that argument, and Terraform refuses to plan until it is supplied.

What this error means

plan or validate stops with "Error: Missing required argument" and "The argument \"X\" is required, but no definition was found." pointing at a module block.

Error: Missing required argument

  on main.tf line 5, in module "network":
   5: module "network" {

The argument "cidr_block" is required, but no definition was found.

Common causes

A required module input was not passed

The module defines variable "cidr_block" {} with no default, so every caller must set it in the module block.

A module upgrade added a new required input

A newer module version introduced a required variable that the existing module block does not yet provide.

How to fix it

Pass the required argument

1. Open the module and list variables without a default.
2. Set each one in the module block.
3. Re-run plan to confirm no required arguments remain.

module "network" {
  source     = "./modules/network"
  cidr_block = "10.0.0.0/16"
}

Give the module input a default where appropriate

If a value is optional, add a default to the module's variable so callers are not forced to set it.

variable "cidr_block" {
  type    = string
  default = "10.0.0.0/16"
}

How to prevent it

• Set all no-default module inputs in every module block.
• Review module upgrade changelogs for new required inputs.
• Run terraform validate in CI to catch missing arguments early.

Frequently asked questions

Related guides

References

• Terraform documentation
• GitHub Actions documentation