Error: Invalid template interpolation value: "Cannot include the given value in a string template"

TL;DR

You put a null (or a non-string like an object) inside ${ ... }. String templates only accept strings. Guard with a default: "${var.name != null ? var.name : ""}" or coalesce(var.name, ""), then re-plan.

The error

Error: Invalid template interpolation value

  on main.tf line 6, in locals:
   6:   templated = "foo=${var.foo}"
     ├────────────────
     │ var.foo is null

The expression result is null. Cannot include a null value in a string
template.

Another common shape:

Error: Invalid template interpolation value: Cannot include the given value in a string template: string required.

Steps to fix

1. Find the interpolation the error names and check the value: the ├ line tells you it is null or an object.

• Expected: you see var.foo is null or similar.

1. Decide what the string should be when the value is absent. Usual fixes:

   templated = "foo=${coalesce(var.foo, "default")}"

or for preconditions/error messages, render nulls explicitly:

   error_message = "tag must match spec (got ${coalesce(local.user_tag, "[unset]")})."

• Expected: the template always receives a string.

1. If you interpolated a whole object ("${module.foo}"), pick the specific attribute instead (module.foo.id).

• Expected: scalar value in the template.

1. Run terraform validate then plan.

• Expected: both pass.

When to use this

• plan/validate fails with Invalid template interpolation value on a ${ } sequence, very often inside error_message strings of preconditions or in templatefile vars.

When NOT to use this

• Invalid function argument is a function-call type problem, not a template problem. terraform validate does NOT catch this (it reports success while plan fails), so always re-plan after fixing.

Compatibility

• All Terraform versions. Note: validate passes while plan fails for this error, a known sharp edge.

Root cause

String templates require every interpolated value to be a string (or null-free convertible). Terraform renders error_message before evaluating the condition, so even a passing precondition dies if its message interpolates a null. Optional variables default to null, which is why this bites optional inputs so often.

Edge cases

• nullable = false on the variable prevents null at the boundary, but the error can still come from locals or data sources.
• join() with a null in the list fails the same way; filter nulls first.
• In templatefile, pass pre-stringified values (tostring()) rather than raw objects.