hashicorp/terraform · critical

unknown schema type to serialize

Error message

unknown schema type to serialize

What it means

Panics in SerializeValueForHash (serialize.go:78) default branch. The outer switch serializes by schema.Type and only handles the defined ValueType constants; an uninitialized or out-of-range Type (e.g. TypeInvalid) panics.

Source

Thrown at internal/legacy/helper/schema/serialize.go:78

				buf.WriteString(strconv.FormatFloat(innerVal, 'g', -1, 64))
			case string:
				buf.WriteString(innerVal)
			default:
				panic(fmt.Sprintf("unknown value type in TypeMap %T", innerVal))
			}

			buf.WriteRune(';')
		}
		buf.WriteRune(']')
	case TypeSet:
		buf.WriteRune('{')
		s := val.(*Set)
		for _, innerVal := range s.List() {
			serializeCollectionMemberForHash(buf, innerVal, schema.Elem)
		}
		buf.WriteRune('}')
	default:
		panic("unknown schema type to serialize")
	}
	buf.WriteRune(';')
}

// SerializeValueForHash appends a serialization of the given resource config
// to the given buffer, guaranteeing deterministic results given the same value
// and schema.
//
// Its primary purpose is as input into a hashing function in order
// to hash complex substructures when used in sets, and so the serialization
// is not reversible.
func SerializeResourceForHash(buf *bytes.Buffer, val interface{}, resource *Resource) {
	if val == nil {
		return
	}
	sm := resource.Schema
	m := val.(map[string]interface{})
	var keys []string

View on GitHub (pinned to c9def3e214)

Solutions

1. Set schema.Type to a defined constant on every schema reachable from a TypeSet.
2. Add a schema validation pass asserting Type != TypeInvalid before exercising set hashing.

Example fix

// before
"items": { Type: schema.TypeSet, Elem: &schema.Schema{} },
// after
"items": { Type: schema.TypeSet, Elem: &schema.Schema{Type: schema.TypeString} },

for _, s := range schemaReachableFromSets(r) {
    if !knownValueType(s.Type) { return fmt.Errorf("set element has unknown Type") }
}

func knownValueType(t schema.ValueType) bool {
    switch t {
    case schema.TypeBool, schema.TypeInt, schema.TypeFloat, schema.TypeString,
        schema.TypeList, schema.TypeMap, schema.TypeSet:
        return true
    }
    return false
}

When it happens

Trigger: A schema element that is part of a TypeSet being hashed, whose Type is TypeInvalid or an unknown constant.

Common situations: Same root cause as 981/986: a schema entry missing Type, used inside a set so it reaches the hashing serializer.

Related errors

• unknown value type in TypeMap %T
• invalid element type: %T
• missing field in set: %s.%s
• set item just set doesn't exist
• invalid set element type

AI-assisted analysis of hashicorp/terraform@c9def3e214 (2026-08-07). Data as JSON: /api/errors/9967d88ce36c74b8. Report an issue: GitHub.