diff --git a/CHANGELOG.md b/CHANGELOG.md
index da9a440..b1006a7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,10 +25,10 @@
   will not match the sender it came from. Ids most SMSCs issue are digits or hex and are unaffected.
 - A `U+0000` inside a C-Octet String — `source_addr`, `message_id`, `system_id` and the rest — is
   refused. An Octet String carries a NULL as before.
-- A number with no decimal spelling — `NaN`, `Infinity`, `-Infinity` — is refused where a text field
-  takes one. `sendSms({ from: NaN })` put the literal sender `NaN` on the wire and reported the send
-  as successful; `message_id`, `source_addr` and every other text field took such a number the same
-  way. A finite number still writes its decimals, so `message_id: 123` is unchanged.
+- A non-finite number — `NaN`, `Infinity`, `-Infinity` — is refused where a text field takes one.
+  `sendSms({ from: NaN })` put the literal sender `NaN` on the wire and reported the send as
+  successful; `message_id`, `source_addr` and every other text field took such a number the same way.
+  Any other number goes out as `String()` spells it, so `message_id: 123` is unchanged.
 
 ## 0.5.0
 
diff --git a/test/pdu.test.ts b/test/pdu.test.ts
index e4563dc..cab91b7 100644
--- a/test/pdu.test.ts
+++ b/test/pdu.test.ts
@@ -51,6 +51,11 @@ describe('header', () => {
 		assert.equal(decode(encode({ cmdName: 'enquire_link', seqNr: 0x80000001 })).seqNr, 0x80000001);
 		assert.equal(decode(encode({ cmdName: 'enquire_link', seqNr: 0xFFFFFFFF })).seqNr, 0xFFFFFFFF);
 		assert.ok(objToPdu({ cmdName: 'submit_sm', seqNr: 0x100000000 }).err instanceof Error);
+
+		const notANumber = objToPdu({ cmdName: 'submit_sm', seqNr: NaN });
+
+		assert.ok(notANumber.err instanceof Error);
+		assert.match(notANumber.err.message, /NaN/);
 	});
 });
 
@@ -144,7 +149,7 @@ describe('parsing real PDUs', () => {
 		assert.equal(decode(pdu).params.source_addr, 'Kaffeé');
 	});
 
-	test('refuses an address the field cannot carry rather than truncating it', () => {
+	test('refuses an address the field cannot carry rather than truncating or coercing it', () => {
 		const smuggled = objToPdu({
 			cmdName: 'submit_sm',
 			params: { destination_addr: '46709771337', source_addr: '46701113311\u0000EVIL' },
@@ -154,7 +159,6 @@ describe('parsing real PDUs', () => {
 		assert.equal(smuggled.buffer, undefined);
 		assert.ok(objToPdu({ cmdName: 'deliver_sm', params: { source_addr: '一' } }).err instanceof Error);
 
-		// Coercing these spells the senders "NaN" and "Infinity", which a peer reads as those letters.
 		assert.ok(objToPdu({ cmdName: 'deliver_sm', params: { source_addr: NaN } }).err instanceof Error);
 		assert.ok(objToPdu({ cmdName: 'deliver_sm', params: { source_addr: Infinity } }).err instanceof Error);
 	});
diff --git a/test/types.test.ts b/test/types.test.ts
index 51ddc51..c55cc86 100644
--- a/test/types.test.ts
+++ b/test/types.test.ts
@@ -39,6 +39,12 @@ describe('integers', () => {
 		assert.ok(types.int8.write(-1, Buffer.alloc(1), 0).err instanceof Error);
 		assert.ok(types.int16.write(1.5, Buffer.alloc(2), 0).err instanceof Error);
 		assert.ok(types.int8.write('nope', Buffer.alloc(1), 0).err instanceof Error);
+
+		const notANumber = types.int8.write(NaN, Buffer.alloc(1), 0);
+
+		// JSON spells NaN and the infinities `null`, which is a value the caller never wrote.
+		assert.ok(notANumber.err instanceof Error);
+		assert.match(notANumber.err.message, /NaN/);
 	});
 });
 
@@ -112,7 +118,7 @@ describe('cstring (C-Octet String)', () => {
 		assert.deepEqual(target, encoded);
 	});
 
-	test('coerces a numeric value to its decimal string, and refuses one with no decimals', () => {
+	test('coerces a numeric value to its decimal string, and refuses a non-finite one', () => {
 		const target = Buffer.alloc(4);
 
 		types.cstring.write(123, target, 0);
@@ -124,7 +130,9 @@ describe('cstring (C-Octet String)', () => {
 			assert.ok(types.cstring.size(value).err instanceof Error, String(value));
 			assert.ok(types.cstring.write(value, Buffer.alloc(9), 0).err instanceof Error, String(value));
 			assert.ok(types.string.write(value, Buffer.alloc(9), 0).err instanceof Error, String(value));
+			assert.ok(types.buffer.write(value, Buffer.alloc(9), 0).err instanceof Error, String(value));
 			assert.ok(types.tlv.string.write(value, Buffer.alloc(9), 0).err instanceof Error, String(value));
+			assert.ok(types.tlv.cstring.write(value, Buffer.alloc(9), 0).err instanceof Error, String(value));
 		}
 	});