hashicorp/terraform · error

exactly one argument is required

Error message

exactly one argument is required

What it means

encode_tfvars builtin guard: the function schema declares exactly one parameter, so the language runtime should reject zero-arg calls before this code runs. The check is purely defensive robustness; reaching it implies the caller bypassed schema validation.

Solutions

1. Call encode_tfvars through normal HCL so the schema enforces arity (encode_tfvars(var.obj)).
2. If calling programmatically, supply exactly one argument matching the declared parameter.
3. Ensure the function's declared parameter count matches when wrapping/monkey-patching.

Example fix

// before (programmatic misuse)
encodeTfvarsFunc([]cty.Value{})
// after
encodeTfvarsFunc([]cty.Value{cty.ObjectVal(map[string]cty.Value{"x": cty.StringVal("y")})})

// Programmatic callers: enforce arity before dispatch
if len(args) != 1 { return cty.NilVal, fmt.Errorf("encode_tfvars takes exactly 1 arg") }

func encodeTfvarsArityOK(args []cty.Value) bool { return len(args) == 1 }

When it happens

Trigger: encodeTfvarsFunc invoked with len(args) == 0: a direct programmatic call to the function without args (not via HCL), or a bug in the function dispatcher that skipped schema-based arity checks.

Common situations: Internal testing invoking the function directly; a regression in the function dispatcher; experimental code path bypassing the schema layer.

Related errors

• argument must be a string
• action schema not found for action
• Can't serialize backend configuration as JSON
• error writing state
• Failed to marshal state to json

AI-assisted analysis of hashicorp/terraform@d32a084675 (2026-08-11). Data as JSON: /api/errors/213672c840b8a221. Report an issue: GitHub.

Appendix: source

Thrown at internal/builtin/providers/terraform/functions.go:32

	"github.com/zclconf/go-cty/cty"
	"github.com/zclconf/go-cty/cty/function"
)

var functions = map[string]func([]cty.Value) (cty.Value, error){
	"encode_tfvars": encodeTfvarsFunc,
	"decode_tfvars": decodeTfvarsFunc,
	"encode_expr":   encodeExprFunc,
}

func encodeTfvarsFunc(args []cty.Value) (cty.Value, error) {
	// These error checks should not be hit in practice because the language
	// runtime should check them before calling, so this is just for robustness
	// and completeness.
	if len(args) > 1 {
		return cty.NilVal, function.NewArgErrorf(1, "too many arguments; only one expected")
	}
	if len(args) == 0 {
		return cty.NilVal, fmt.Errorf("exactly one argument is required")
	}

	v := args[0]
	ty := v.Type()

	if v.IsNull() {
		// Our functions schema does not say we allow null values, so we should
		// not get to this error message if the caller respects the schema.
		return cty.NilVal, function.NewArgErrorf(1, "cannot encode a null value in tfvars syntax")
	}
	if !v.IsWhollyKnown() {
		return cty.UnknownVal(cty.String).RefineNotNull(), nil
	}

	var keys []string
	switch {
	case ty.IsObjectType():
		atys := ty.AttributeTypes()

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