hashicorp/terraform · error

input must be a map of lists of strings

Error message

input must be a map of lists of strings

What it means

Thrown by `transpose()` (TransposeFunc) when an inner-list element's type is not exactly cty.String. transpose() is typed as map(list(string)) and each value becomes a string key in the output map, so non-string elements (numbers, bools) are rejected even though the declared type permits the map(list(...)) shape. The check uses val.Type().Equals(cty.String).

Solutions

1. Convert inner elements to strings: `{ for k, v in var.m : k => [for x in v : tostring(x)] }`.
2. Declare the variable type strictly as map(list(string)) so mismatches surface earlier.
3. Re-type the upstream data source to emit strings.

Example fix

// before
locals { t = transpose(var.ids) }   # var.ids is map(list(any)) with numbers

// after
locals { t = transpose({ for k, v in var.ids : k => [for x in v : tostring(x)] }) }

# HCL: stringify inner elements before transpose
locals { t = transpose({ for k, v in var.m : k => [for x in v : tostring(x)] }) }

# HCL: declare strictly typed variable
variable "m" { type = map(list(string)) }

When it happens

Trigger: Calling transpose with a map whose inner lists hold non-strings, e.g. `transpose({a = [1, 2]})`. A variable declared loosely as map(list(any)) fed numeric values.

Common situations: Loosely typed variables (map(list(any))) that happen to carry numbers. Data from a datasource where the inner element type is number rather than string.

Related errors

• argument must be a list or tuple
• argument must be a string, a collection type, or a…
• input list must not contain null string
• input must not contain null list
• all arguments must have the same type

AI-assisted analysis of hashicorp/terraform@d32a084675 (2026-08-11). Data as JSON: /api/errors/f97dbc2f224a9e5d. Report an issue: GitHub.

Appendix: source

Thrown at internal/lang/funcs/collection.go:594

		if !inputMap.IsWhollyKnown() {
			return cty.UnknownVal(retType), nil
		}

		outputMap := make(map[string]cty.Value)
		tmpMap := make(map[string][]string)

		for it := inputMap.ElementIterator(); it.Next(); {
			inKey, inVal := it.Element()
			if inVal.IsNull() {
				return cty.MapValEmpty(cty.List(cty.String)), errors.New("input must not contain null list")
			}
			for iter := inVal.ElementIterator(); iter.Next(); {
				_, val := iter.Element()
				if val.IsNull() {
					return cty.MapValEmpty(cty.List(cty.String)), errors.New("input list must not contain null string")
				}
				if !val.Type().Equals(cty.String) {
					return cty.MapValEmpty(cty.List(cty.String)), errors.New("input must be a map of lists of strings")
				}

				outKey := val.AsString()
				if _, ok := tmpMap[outKey]; !ok {
					tmpMap[outKey] = make([]string, 0)
				}
				outVal := tmpMap[outKey]
				outVal = append(outVal, inKey.AsString())
				sort.Strings(outVal)
				tmpMap[outKey] = outVal
			}
		}

		for outKey, outVal := range tmpMap {
			values := make([]cty.Value, 0)
			for _, v := range outVal {
				values = append(values, cty.StringVal(v))
			}

View on GitHub (pinned to d32a084675)