diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0a9f2d5..1606b0b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -35,6 +35,8 @@
 - An `alert_notification` or an `outbind` from the peer is logged and left unanswered, as SMPP 3.4
   gives neither a response. Each one used to emit `sessionError`, `"alert_notification" has no
   response command`.
+- `server()` refuses a `maxOctets` below 1 or not a whole number, `Infinity` included, like its
+  other limits. `server({ maxOctets: 0 })` used to start and then refuse every multipart message.
 
 ## 0.5.0
 
diff --git a/src/reassembly.ts b/src/reassembly.ts
index bacadcb..98a7abc 100644
--- a/src/reassembly.ts
+++ b/src/reassembly.ts
@@ -43,7 +43,7 @@ export type Collected =
 		whole?: PduObject[] | undefined;
 	};
 
-const defaultMaxOctets = 64 * 1024 * 1024;
+export const defaultMaxOctets = 64 * 1024 * 1024;
 
 type Group = {
 	octets: number;
diff --git a/src/session-options.ts b/src/session-options.ts
index fd53842..ea1dbf0 100644
--- a/src/session-options.ts
+++ b/src/session-options.ts
@@ -9,6 +9,7 @@ import type { SmsIdFormat } from './sms-id.ts';
 import type { Sms } from './sms.ts';
 import type { Socket } from 'node:net';
 import { backoffDefaults } from './reconnect-loop.ts';
+import { defaultMaxOctets } from './reassembly.ts';
 import { isSmsIdNotation, smsIdNotations, smsIdPlaces } from './sms-id.ts';
 import { namedValue } from './error-from.ts';
 
@@ -160,6 +161,7 @@ export function checkSessionOptions(options: CheckableOptions): VoidResult {
 function limitsOf(options: CheckableOptions): [string, number, number][] {
 	return [
 		['idleTimeout', options.idleTimeout ?? 0, 0],
+		['maxOctets', options.maxOctets ?? defaultMaxOctets, 1],
 		['maxOutstanding', options.maxOutstanding ?? defaults.maxOutstanding, 1],
 		['maxReassembly', options.maxReassembly ?? defaults.maxReassembly, 1],
 		['reassemblyTimeout', options.reassemblyTimeout ?? defaults.reassemblyTimeout, 0],
@@ -268,6 +270,7 @@ export type CheckableOptions = {
 	/** Not an option: the one spelling is inside reconnect, and this is where the other is refused. */
 	fromStart?: unknown;
 	idleTimeout?: number | undefined;
+	maxOctets?: number | undefined;
 	maxOutstanding?: number | undefined;
 	maxReassembly?: number | undefined;
 	reassemblyTimeout?: number | undefined;
diff --git a/test/session.test.ts b/test/session.test.ts
index 3b4de31..5e936ae 100644
--- a/test/session.test.ts
+++ b/test/session.test.ts
@@ -12,6 +12,7 @@ import { DlrMerger } from '../src/dlr-merger.ts';
 import { PduFramer } from '../src/pdu-framer.ts';
 import { ReconnectLoop } from '../src/reconnect-loop.ts';
 import { Session, bindCommands } from '../src/session.ts';
+import { checkSessionOptions } from '../src/session-options.ts';
 import { client } from '../src/client.ts';
 import { closeAfter, closeListenerAfter } from './teardown.ts';
 import { consts } from '../src/defs/constants.ts';
@@ -2688,6 +2689,15 @@ describe('option validation', () => {
 		assert.match(hookRefused.err?.message ?? '', /onRequest must be a function/);
 	});
 
+	test('refuses a reassembly octet cap below 1 at startup', async () => {
+		const listening = await server({ maxOctets: 0, port: 0 });
+
+		if (listening.server) await listening.server.close();
+
+		assert.match(listening.err?.message ?? '', /maxOctets must be 1 or more, got 0/);
+		assert.match(checkSessionOptions({ maxOctets: Infinity }).err?.message ?? '', /maxOctets must be 1 or more, got Infinity/);
+	});
+
 	test('returns an error rather than rejecting on an impossible port', async () => {
 		const listening = await server({ port: 70_000 });
 
diff --git a/todo.md b/todo.md
index 25bc6fd..1276023 100644
--- a/todo.md
+++ b/todo.md
@@ -189,12 +189,6 @@ and is also what the panel ranked hardest — two methods, one answer.
 
 ### Correctness, ahead of everything below
 
-- [ ] **Range-check `maxOctets` with its five siblings.** `limitsOf()` in `session-options.ts`
-      covers `idleTimeout`, `maxOutstanding`, `maxReassembly`, `reassemblyTimeout`, `responseTimeout`
-      and `shutdownTimeout`; `maxOctets` is documented, consumed by `Reassembler`, and absent from
-      both that list and `CheckableOptions`. `server({ maxOctets: 0 })` starts, then refuses every
-      multipart message and reports each as lost traffic.
-
 - [ ] **Read `multiple` in `parseTlvs()` and `writeTlvs()`, or delete it and `tlvMap`.** Five TLVs
       declare `multiple: true` (`callback_num`, `callback_num_atag`, `callback_num_pres_ind`,
       `broadcast_area_identifier`, `broadcast_error_status`) and nothing reads it; `parseTlvs()` keys