hashicorp/terraform · error

all arguments must have the same type

Error message

all arguments must have the same type

What it means

Thrown by CoalesceFunc.Type (the HCL coalesce() builtin). coalesce() must return a single type, so all arguments are passed through convert.UnifyUnsafe; if they cannot be unified it reports that all arguments must share a type.

Source

Thrown at internal/lang/funcs/collection.go:152

// stdlib and modified so that it returns the first *non-empty* non-null element
// from a sequence, instead of merely the first non-null.
var CoalesceFunc = function.New(&function.Spec{
	Params: []function.Parameter{},
	VarParam: &function.Parameter{
		Name:             "vals",
		Type:             cty.DynamicPseudoType,
		AllowUnknown:     true,
		AllowDynamicType: true,
		AllowNull:        true,
	},
	Type: func(args []cty.Value) (ret cty.Type, err error) {
		argTypes := make([]cty.Type, len(args))
		for i, val := range args {
			argTypes[i] = val.Type()
		}
		retType, _ := convert.UnifyUnsafe(argTypes)
		if retType == cty.NilType {
			return cty.NilType, errors.New("all arguments must have the same type")
		}
		return retType, nil
	},
	RefineResult: refineNotNull,
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		for _, argVal := range args {
			// We already know this will succeed because of the checks in our Type func above
			argVal, _ = convert.Convert(argVal, retType)
			if !argVal.IsKnown() {
				return cty.UnknownVal(retType), nil
			}
			if argVal.IsNull() {
				continue
			}
			if retType == cty.String && argVal.RawEquals(cty.StringVal("")) {
				continue
			}

View on GitHub (pinned to c9def3e214)

Solutions

1. Normalize each argument to the desired type before calling: coalesce(tostring(var.a), tostring(var.b)).
2. Pick arguments that are inherently the same type.
3. Use try() with explicit conversions when types are uncertain.

Example fix

# before
locals { name = coalesce(var.display_name, var.id) }   # string vs number
# after
locals { name = coalesce(var.display_name, tostring(var.id)) }

# normalize all coalesce args to one type beforehand
locals { name = coalesce(tostring(var.a), tostring(var.b)) }

# guard: unify types before calling coalesce
locals {
  a_str = try(tostring(var.a), null)
  b_str = try(tostring(var.b), null)
  name  = coalesce(a_str, b_str)
}

locals { name = try(coalesce(tostring(var.a), tostring(var.b)), "default") }

When it happens

Trigger: Mixing types in one call: coalesce("a", 1), coalesce([], {}), coalesce(var.s, var.n) where one is string and one number.

Common situations: Coalescing a string default with a numeric value, or two differently-typed optional variables; forgetting that '0' vs 0 differ in type.

Related errors

• argument must be a string, a collection type, or a structura
• no non-null, non-empty-string arguments
• argument must be a list or tuple
• keys and searchset must be of the same type
• input must be a map of lists of strings

AI-assisted analysis of hashicorp/terraform@c9def3e214 (2026-08-07). Data as JSON: /api/errors/1368fd7abd8cc4e6. Report an issue: GitHub.