diff --git a/CHANGELOG.md b/CHANGELOG.md
index bb3bb43..02e45dd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -72,7 +72,7 @@
   A decimal key naming a tag the table knows, `{ 1063: … }`, is refused in favour of the name, and
   so are `alert_on_msg_delivery` and `failed_broadcast_area_identifier` in favour of
   `alert_on_message_delivery` and `broadcast_area_identifier`, the names they read back under. The
-  two alternate names are gone from `tlvs` too, which is now typed by `TlvName`: index it with one.
+  two alternate names are gone from `tlvs` too, which is now typed by `TlvName`: narrow a `string` with `isTlvName()` before indexing it.
 - `cmds.broadcast_sm_resp.tlvMap` is removed; nothing read it.
 
 ## 0.5.0
diff --git a/README.md b/README.md
index 864aa30..ba17dc9 100644
--- a/README.md
+++ b/README.md
@@ -655,7 +655,7 @@ if (isCommand(pduObj, 'submit_sm')) {
 | Messages | `encodeMessage`, `decodeMessage`, `splitMessage`, `bitCount`, `messageOctets`, `concatOf`, `concatInfo`, `detect`, `unencodable`, `messageClassOf`, `dataCodingByEncoding`, `encodingByDataCoding` |
 | Receipts | `dlrFromPdu`, `parseReceipt`, `receiptCodes` |
 | Time and ids | `smppDate`, `smppTime`, `uuidv7` |
-| Spec tables | `cmds`, `consts`, `encodings`, `errors`, `tlvs`, `types`, the `cmdsById`, `constsById`, `errorsById` and `tlvsById` maps, and all of them grouped as `defs`. `isCommandName`, `isErrorName`, `isEncodingName`, `commandNameById` and `errorNameById` narrow a value into them. |
+| Spec tables | `cmds`, `consts`, `encodings`, `errors`, `tlvs`, `types`, the `cmdsById`, `constsById`, `errorsById` and `tlvsById` maps, and all of them grouped as `defs`. `isCommandName`, `isErrorName`, `isEncodingName`, `isTlvName`, `commandNameById` and `errorNameById` narrow a value into them. |
 | Types | Every option, result, event payload and table entry has a named type: `ClientOptions`, `ServerOptions`, `SendSmsOptions`, `SendSmsResult`, `Sms`, `Dlr`, `MessageDlr`, `Receipt`, `PduObject`, `PduHeader`, `SmppLog`, `Result` and the rest in `dist/index.d.ts`. |
 
 ## What changed per release
diff --git a/src/defs/tlvs.ts b/src/defs/tlvs.ts
index c48ef40..fc4d6b0 100644
--- a/src/defs/tlvs.ts
+++ b/src/defs/tlvs.ts
@@ -253,7 +253,7 @@ function readTlv(pdu: Buffer, offset: number): Result<{ octets: number; occurren
 	return { occurrence: { definition, tagId, value }, octets: 4 + tagLength };
 }
 
-function isTlvName(name: string): name is TlvName {
+export function isTlvName(name: string): name is TlvName {
 	return Object.hasOwn(specs, name);
 }
 
diff --git a/src/index.ts b/src/index.ts
index 837f3f0..509250a 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -6,7 +6,7 @@ export { cmds, cmdsById, commandNameById, isCommandName } from './defs/commands.
 export { consts, constsById } from './defs/constants.ts';
 export { dataCodingByEncoding, detect, encodingByDataCoding, encodings, isEncodingName, messageClassOf, unencodable } from './defs/encodings.ts';
 export { errorNameById, errors, errorsById, isErrorName } from './defs/errors.ts';
-export { tlvs, tlvsById } from './defs/tlvs.ts';
+export { isTlvName, tlvs, tlvsById } from './defs/tlvs.ts';
 export { types } from './defs/types.ts';
 
 export {
diff --git a/test/pdu.test.ts b/test/pdu.test.ts
index 06d60b8..c773865 100644
--- a/test/pdu.test.ts
+++ b/test/pdu.test.ts
@@ -3,7 +3,7 @@ import test, { describe } from 'node:test';
 import { PduRefusedError, refusalAnswer } from '../src/pdu-refusal.ts';
 import { isCommand, isResp, objToPdu, pduReturn, pduToObj } from '../src/pdu.ts';
 import { paramText } from '../src/defs/types.ts';
-import { tlvsById } from '../src/defs/tlvs.ts';
+import { isTlvName, tlvsById } from '../src/defs/tlvs.ts';
 
 function encode(...args: Parameters<typeof objToPdu>): Buffer {
 	const { buffer, err } = objToPdu(...args);
@@ -502,6 +502,10 @@ describe('TLVs', () => {
 		const { err } = objToPdu({ cmdName: 'broadcast_sm_resp', params, tlvs: { failed_broadcast_area_identifier: { tagValue: areas } } });
 
 		assert.match(err?.message ?? '', /key it broadcast_area_identifier/);
+		assert.deepEqual(
+			['broadcast_area_identifier', 'failed_broadcast_area_identifier', 'constructor'].map(isTlvName),
+			[true, false, false],
+		);
 	});
 
 	test('refuses a repeatable TLV given one value, and a lone TLV given several', () => {