How to Fix 'The Given Value Cannot Be Converted to a String' in Terraform Locals

Threat/Impact Level: HIGH | Exploitability/Downtime Risk: HIGH | Time to Fix: 5–15 mins

TL;DR

• What broke: A locals block is assigning a non-string value (object, list, bool, null, or a complex expression) to a variable that Terraform's type system cannot silently coerce into string.
• How to fix it: Wrap the offending value with an explicit conversion function—tostring(), jsonencode(), or join()—matching the actual data shape.
• Fast path: Use our Client-Side Sandbox above to paste your failing locals block and auto-refactor it without leaking your config anywhere.

---

The Incident — What Does the Error Mean?

Raw error output from terraform plan or terraform apply:

Error: The given value cannot be converted to a string

  on main.tf line 14, in locals:
  14:   env_tag = var.environment_config

The given value is an object type, and cannot be converted to string.

Terraform's type system is strict at evaluation time, not at parse time. The HCL evaluator walks every locals expression and attempts to resolve the declared or inferred type. When the right-hand side resolves to object, list, tuple, bool, or null—and the consuming resource attribute, output, or another local explicitly or implicitly demands string—the engine panics with this error and halts the entire plan. No partial apply. No state update. Full stop.

Immediate consequence: Your pipeline is blocked. Every downstream module, resource, or output that references this local is also dead until the type is resolved.

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The Attack Vector / Blast Radius

This is a plan-time type-system failure, not a runtime warning. The blast radius is:

1. Full pipeline halt. terraform plan exits non-zero. Any CI/CD gate (GitHub Actions, GitLab CI, Atlantis, Spacelift) marks the job failed and blocks the PR merge.
2. Cascading local failures. Terraform evaluates all locals in dependency order. One broken local that other locals reference causes a chain reaction—you may see this single error masking 5–10 downstream failures that only surface after you fix the first.
3. State drift window. If this error surfaces mid-refactor on a live workspace, your actual infrastructure and your state file may already be partially diverged. Every minute the plan is broken is a minute your team is flying blind.
4. Module boundary explosion. If the broken local is exported via output to a calling module, the calling module's plan also fails—even if the calling module's own code is perfectly valid.

The most common triggers in production:

Root Cause	Example
Passing a map/object variable directly to a string local	local.tag = var.tags where var.tags is map(string)
Boolean flag used as a string label	local.flag = var.enable_feature where type is bool
null default not guarded	local.name = var.override_name where var.override_name defaults to null
Splat or index expression returning a tuple	local.first_id = aws_subnet.main[*].id
merge() or lookup() returning an object	local.config_str = merge(local.base, local.override)

---

How to Fix It

Basic Fix — Explicit Type Conversion

The fastest resolution is wrapping the offending value with the correct Terraform built-in conversion function.

Scenario 1: Object or map passed as string

locals {
-  env_tag = var.environment_config
+  env_tag = jsonencode(var.environment_config)
}

Scenario 2: Boolean used as string label

locals {
-  feature_flag_label = var.enable_feature
+  feature_flag_label = tostring(var.enable_feature)
}

Scenario 3: List or tuple coerced to string

locals {
-  subnet_ids_str = aws_subnet.main[*].id
+  subnet_ids_str = join(",", aws_subnet.main[*].id)
}

Scenario 4: Null-guarded string with fallback

locals {
-  display_name = var.override_name
+  display_name = var.override_name != null ? var.override_name : "default-name"
}

---

Enterprise Best Practice — Typed Variables with Explicit Contracts

Don't rely on conversion functions as a patch. Enforce types at the variable declaration boundary so the error is impossible to introduce.

# variables.tf
variable "environment_config" {
-  type = any
+  type = object({
+    name        = string
+    tier        = string
+    cost_center = string
+  })
+  description = "Structured environment metadata. Use environment_config.name for string locals."
}

# locals.tf
locals {
-  env_tag = var.environment_config
+  env_tag      = var.environment_config.name
+  env_tag_json = jsonencode(var.environment_config)
}

Why this matters at scale:

• type = any is the #1 root cause of this error in shared module registries. It defers type resolution until consumption, making failures non-local and hard to trace.
• Typed variable declarations give you free validation via terraform validate in CI before plan is ever invoked.
• Explicit attribute access (var.environment_config.name) is always safer than passing the whole object and hoping downstream coercion works.

---

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---

Prevention in CI/CD

Fix it once in prod. Never again in the pipeline.

1. terraform validate as a Pre-Plan Gate

# .github/workflows/terraform.yml
- name: Terraform Validate
  run: terraform validate
  # Catches type errors before plan is ever invoked.
  # Zero cost. Zero excuses for skipping this.

2. TFLint with the AWS/AzureRM Ruleset

# .tflint.hcl
plugin "terraform" {
  enabled = true
  version = "0.5.0"
  source  = "github.com/terraform-linters/tflint-ruleset-terraform"
}

rule "terraform_typed_variables" {
  enabled = true  # Flags every `type = any` declaration
}

tflint --recursive in your CI pipeline will flag every type = any variable before it can cause a plan-time failure.

3. Checkov Policy — Ban type = any

# checkov custom policy: ban_type_any.py
from checkov.terraform.checks.resource.base_resource_check import BaseResourceCheck

# Or use the built-in OPA approach with a Rego policy:
# Deny any variable block where type == "any"

OPA Rego (for Terraform plan JSON):

package terraform.locals

deny[msg] {
  v := input.variables[name]
  v.type == "any"
  msg := sprintf("Variable '%s' uses type=any. Declare an explicit type to prevent string coercion errors.", [name])
}

4. Pre-Commit Hook

# .pre-commit-config.yaml
repos:
  - repo: https://github.com/antonbabenko/pre-commit-terraform
    rev: v1.88.0
    hooks:
      - id: terraform_validate
      - id: terraform_tflint
        args:
          - --args=--config=__GIT_WORKING_DIR__/.tflint.hcl

This catches the error on the developer's machine before a single commit reaches the remote, eliminating the CI feedback loop entirely.