diff --git a/test/session.test.ts b/test/session.test.ts
index d50de32..30a1e4e 100644
--- a/test/session.test.ts
+++ b/test/session.test.ts
@@ -455,15 +455,23 @@ describe('bind', () => {
 		assert.equal(session.boundAs, undefined);
 		assert.equal(session.peerInterfaceVersion, undefined);
 
-		session.bound('receiver', 0x34);
-
+		assert.deepEqual(session.bound('receiver', 0x34), {});
 		assert.equal(session.boundAs, 'receiver');
 		assert.equal(session.peerInterfaceVersion, 0x34);
 		assert.equal(session.bindAllows('submit_sm'), false);
 
-		session.bound('transmitter', undefined);
-
+		assert.deepEqual(session.bound('transmitter', undefined), {});
 		assert.equal(session.peerInterfaceVersion, 0x00, 'no declared version is pre-3.4');
+
+		// The mistakes an untyped caller makes: the TLV object for its value, a version past int8, a typo.
+		for (const [bindType, declared] of [['receiver', { tagValue: 0x34 }], ['receiver', 0x100], ['receiver', 3.4], ['tx', 0x34]]) {
+			const refused: unknown = Reflect.apply(session.bound, session, [bindType, declared]);
+
+			assert.ok(refused && typeof refused === 'object' && 'err' in refused && refused.err instanceof Error);
+		}
+
+		assert.equal(session.boundAs, 'transmitter', 'a refused bind leaves the recorded one alone');
+		assert.equal(session.peerInterfaceVersion, 0x00);
 		assert.equal(Reflect.set(session, 'boundAs', 'transceiver'), false);
 		assert.equal(Reflect.set(session, 'peerInterfaceVersion', 0x50), false);
 		assert.equal(session.boundAs, 'transmitter');