Nil UUID

The all-zero UUID. RFC-defined sentinel, not a generated value.

The literal value

00000000-0000-0000-0000-000000000000

What's special about it

The nil UUID is reserved by RFC 4122. No legitimate generator will ever produce it (the version and variant bits are required to be non-zero), which makes it a safe sentinel — you can be sure that a value of all zeros was deliberately set, not accidentally generated.

The max UUID (FF…FF)

RFC 9562 added a counterpart, the max UUID, all bits set: ffffffff-ffff-ffff-ffff-ffffffffffff. It's useful as an upper bound in range queries when you want "everything up to (but not including) any real UUID."

Common pitfalls

• Some libraries treat nil as "null" in serialization. Check before storing it as a real value.
• Don't use nil as a default in a UNIQUE column — you'll get constraint violations.
• Don't compare against nil with string equality before normalizing case. Most libraries emit lowercase, but a hand-written constant might be uppercase.
• Some ORMs serialize nil as NULL in JSON. If a downstream service expects the literal string, you'll see surprising failures only on the nil case.

Real-world use cases

The nil UUID shows up in a handful of legitimate places:

• Migration sentinels. When backfilling a new uuid column on an existing table, use nil as a temporary "not yet assigned" marker, then update in batches.
• Test fixtures. A constant nil in unit tests makes assertions readable: expect(user.id).toEqual(NIL_UUID) is clearer than a generated value.
• Disabled / disconnected references. A foreign key field meaning "not associated with any parent" can use nil instead of NULL if you need a non-nullable column.
• RPC handshakes. Some protocols send nil during connection setup before a real session UUID is negotiated.

Generating nil in code

// JavaScript
import { NIL } from "uuid";
NIL; // "00000000-0000-0000-0000-000000000000"

// Python
import uuid
str(uuid.UUID(int=0))

// C#
Guid.Empty.ToString()

// Go
import "github.com/google/uuid"
uuid.Nil.String()

// PostgreSQL
SELECT '00000000-0000-0000-0000-000000000000'::uuid;

RFC reference

RFC 4122 §4.1.7 reserves the nil UUID. RFC 9562 (2024) §5.9 reaffirms it and adds the max UUID (§5.10). Both are exempt from version and variant bit checks — a parser that strictly validates those bits should still accept nil and max as special cases.