diff --git a/src/defs/tlvs.ts b/src/defs/tlvs.ts
index 74a557a..4b1033a 100644
--- a/src/defs/tlvs.ts
+++ b/src/defs/tlvs.ts
@@ -166,8 +166,8 @@ function entryOf(name: string, input: unknown): Result<{ tagId: number; tagValue
 
 	if (keyed.err) return { err: keyed.err };
 
-	if ('tagId' in input && input.tagId !== keyed.tagId) {
-		return { err: new Error(`TLV "${name}": tagId ${String(input.tagId)} is not the tag its key names, ${String(keyed.tagId)}; drop the tagId`) };
+	if ('tagId' in input && input.tagId !== undefined && input.tagId !== keyed.tagId) {
+		return { err: new Error(`TLV "${name}": its tagId is not ${String(keyed.tagId)}, the tag its key names; drop the tagId`) };
 	}
 
 	return { tagId: keyed.tagId, tagValue: input.tagValue };
diff --git a/test/pdu.test.ts b/test/pdu.test.ts
index dc373d3..e48e28a 100644
--- a/test/pdu.test.ts
+++ b/test/pdu.test.ts
@@ -443,9 +443,9 @@ describe('TLVs', () => {
 		const refusals = [
 			// @ts-expect-error nils is no tag name
 			{ built: objToPdu({ cmdName: 'deliver_sm', params, tlvs: { nils: { tagValue: 'blajfoo' } } }), reason: /decimal id/ },
-			{ built: objToPdu({ cmdName: 'deliver_sm', params, tlvs: { 5142: { tagId: 5143, tagValue: 'blajfoo' } } }), reason: /tagId 5143/ },
+			{ built: objToPdu({ cmdName: 'deliver_sm', params, tlvs: { 5142: { tagId: 5143, tagValue: 'blajfoo' } } }), reason: /its tagId is not 5142/ },
 			// @ts-expect-error the key names the tag
-			{ built: objToPdu({ cmdName: 'deliver_sm', params, tlvs: { message_state: { tagId: 5, tagValue: 2 } } }), reason: /tagId 5/ },
+			{ built: objToPdu({ cmdName: 'deliver_sm', params, tlvs: { message_state: { tagId: 5, tagValue: 2 } } }), reason: /its tagId is not 1063/ },
 			{ built: objToPdu({ cmdName: 'deliver_sm', params, tlvs: { 5142: { tagValue: 300 } } }), reason: /Buffer/ },
 			{ built: objToPdu({ cmdName: 'deliver_sm', params, tlvs: { 65536: { tagValue: 'blajfoo' } } }), reason: /out of range/ },
 			{ built: objToPdu({ cmdName: 'deliver_sm', params, tlvs: { '05142': { tagValue: 'blajfoo' } } }), reason: /decimal id/ },
@@ -553,6 +553,8 @@ describe('TLVs', () => {
 		}));
 
 		assert.deepEqual(decode(encode({ cmdName: 'deliver_sm', params, tlvs: parsed.tlvs })).tlvs, parsed.tlvs);
+		// @ts-expect-error the key names the tag, and an undefined tagId names none
+		assert.equal(objToPdu({ cmdName: 'deliver_sm', params, tlvs: { message_state: { tagId: undefined, tagValue: 2 } } }).err, undefined);
 	});
 
 	test('types each known TLV by its tag, and an unknown one as octets', () => {