diff --git a/CHANGELOG.md b/CHANGELOG.md
index e2578b6..da9a440 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +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.
 
 ## 0.5.0
 
diff --git a/src/defs/types.ts b/src/defs/types.ts
index 5a24a85..f72e8a3 100644
--- a/src/defs/types.ts
+++ b/src/defs/types.ts
@@ -99,6 +99,10 @@ function wantText(value: ParamValue): Result<{ text: string }> {
 		return { err: new Error(`Expected a string or a number, got ${typeof value}`) };
 	}
 
+	if (typeof value === 'number' && !Number.isFinite(value)) {
+		return { err: new Error(`Expected a finite number, got ${String(value)}`) };
+	}
+
 	const text = String(value);
 
 	return pastLatin1(text) ?? { text };
diff --git a/test/pdu.test.ts b/test/pdu.test.ts
index 7d44959..a774c3b 100644
--- a/test/pdu.test.ts
+++ b/test/pdu.test.ts
@@ -153,6 +153,10 @@ describe('parsing real PDUs', () => {
 		assert.ok(smuggled.err instanceof Error);
 		assert.equal(smuggled.buffer, undefined);
 		assert.ok(objToPdu({ cmdName: 'deliver_sm', params: { source_addr: '一' } }).err instanceof Error);
+
+		// String(NaN) is a sender the peer reads as the three letters, and the send reports success.
+		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 d353335..51ddc51 100644
--- a/test/types.test.ts
+++ b/test/types.test.ts
@@ -112,13 +112,20 @@ describe('cstring (C-Octet String)', () => {
 		assert.deepEqual(target, encoded);
 	});
 
-	test('coerces a numeric value to its decimal string', () => {
+	test('coerces a numeric value to its decimal string, and refuses one with no decimals', () => {
 		const target = Buffer.alloc(4);
 
 		types.cstring.write(123, target, 0);
 
 		assert.deepEqual(target, Buffer.from([0x31, 0x32, 0x33, 0x00]));
 		assert.deepEqual(types.cstring.size(123), { size: 4 });
+
+		for (const value of [NaN, Infinity, -Infinity]) {
+			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.tlv.string.write(value, Buffer.alloc(9), 0).err instanceof Error, String(value));
+		}
 	});
 
 	test('carries every latin1 octet, and refuses a character past it', () => {
diff --git a/todo.md b/todo.md
index e062053..685cf8a 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
 
-- [ ] **Refuse a non-finite number where a text field coerces one.** `wantText()` in `defs/types.ts`
-      stringifies a number so `message_id: 123` writes `"123"`, which is deliberate and tested. It
-      takes `NaN` and `Infinity` on the same path, so `sendSms({ from: NaN })` puts the literal sender
-      `NaN` on the wire and reports success — goal 2. Gate the numeric branch on `Number.isFinite`.
-      Predates the latin1 guard; found by the stability review of #16.
-
 - [ ] **Answer `alert_notification` and `outbind` by not answering them.** Both are response-less in
       SMPP 3.4, both fall through `route()`'s default into `unhandled()`, which calls
       `sendReturn(pduObj, 'ESME_RINVCMDID')`; `pduReturn()` then finds no response command, and the