Invalid function argument Terraform Origin: Platform & cloud OSI layer 7 · Application Last updated 5 Nov 2026

Terraform: Error: Invalid function argument — Invalid value for "list" parameter: list of string required / Call to function "lookup" failed

A built-in function got a wrong type/value (empty list, missing key, wrong type).

Exact message you may see Error: Invalid function argumentInvalid value for "list" parameter: list of string requiredError: Error in function call: Call to function "lookup" failed: the given key does not exist in the given mapError: Invalid function argument Invalid value for "path" parameter: no file exists atCall to function "element" failed: cannot use element function with an empty list
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Seen on: Terraform

Meaning

Read the function and parameter named. Common: element()/[0] on empty lists, lookup() without a default, file() path missing, tonumber("abc"), merge with null, join of non-strings. Test in terraform console. Use try(), coalesce(), lookup(map, key, default), length() guards.

Common causes

  • Indexing an empty list
  • lookup key missing and no default
  • file("path") doesn't exist
  • Type conversion failures
  • Passing null to functions requiring values

⚡ Quick fix

  1. lookup(var.map, key, null) / try(expr, default)
  2. Check lists with length(x) > 0 ? x[0] : null
  3. fileexists() before file()

Detailed fix by platform

Terraform

  1. terraform console then > lookup({a=1}, "b", 0)

How to diagnose

  1. Function — Which?
  2. Argument — Type/value?
  3. Guard — try/default?

🔧 Still not fixed?

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Where this error sits

Origin: Platform & cloud. Errors that are often confused with or caused by this one (click to open):

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