diff --git a/lighter/ws_client.py b/lighter/ws_client.py
index 14abdf5..5ed2929 100644
--- a/lighter/ws_client.py
+++ b/lighter/ws_client.py
@@ -42,6 +42,9 @@ def on_message(self, ws, message):
         if isinstance(message, str):
             message = json.loads(message)
 
+        if not isinstance(message, dict):
+            return
+
         message_type = message.get("type")
 
         if message_type == "connected":
@@ -61,7 +64,12 @@ def on_message(self, ws, message):
             self.handle_unhandled_message(message)
 
     async def on_message_async(self, ws, message):
-        message = json.loads(message)
+        if isinstance(message, str):
+            message = json.loads(message)
+
+        if not isinstance(message, dict):
+            return
+
         message_type = message.get("type")
 
         if message_type == "connected":
diff --git a/test/test_ws_client.py b/test/test_ws_client.py
new file mode 100644
index 0000000..8d49d21
--- /dev/null
+++ b/test/test_ws_client.py
@@ -0,0 +1,21 @@
+import unittest
+
+from lighter.ws_client import WsClient
+
+
+class TestWsClient(unittest.IsolatedAsyncioTestCase):
+    def setUp(self):
+        self.client = WsClient(order_book_ids=[0])
+
+    def test_sync_ignores_json_string_message(self):
+        self.client.on_message(None, '"heartbeat"')
+
+    async def test_async_ignores_json_string_message(self):
+        await self.client.on_message_async(None, '"heartbeat"')
+
+    async def test_async_ignores_decoded_non_object_message(self):
+        await self.client.on_message_async(None, ["heartbeat"])
+
+
+if __name__ == "__main__":
+    unittest.main()