diff --git a/langfuse/_utils/serializer.py b/langfuse/_utils/serializer.py
index 3d795d13e..3c4fad58b 100644
--- a/langfuse/_utils/serializer.py
+++ b/langfuse/_utils/serializer.py
@@ -164,10 +164,26 @@ def _default_inner(self, obj: Any) -> Any:
                 return [self.default(item) for item in obj]
 
             if hasattr(obj, "__slots__"):
-                return {
-                    slot: self.default(getattr(obj, slot, None))
-                    for slot in obj.__slots__
-                }
+                obj_id = id(obj)
+
+                if obj_id in self.seen:
+                    # Break on circular references
+                    return type(obj).__name__
+                else:
+                    self.seen.add(obj_id)
+                    try:
+                        slots = (
+                            (obj.__slots__,)
+                            if isinstance(obj.__slots__, str)
+                            else obj.__slots__
+                        )
+                        return {
+                            slot: self.default(getattr(obj, slot, None))
+                            for slot in slots
+                        }
+                    finally:
+                        self.seen.remove(obj_id)
+
             elif hasattr(obj, "__dict__"):
                 obj_id = id(obj)
 
@@ -176,8 +192,10 @@ def _default_inner(self, obj: Any) -> Any:
                     return type(obj).__name__
                 else:
                     self.seen.add(obj_id)
-                    result = {k: self.default(v) for k, v in vars(obj).items()}
-                    self.seen.remove(obj_id)
+                    try:
+                        result = {k: self.default(v) for k, v in vars(obj).items()}
+                    finally:
+                        self.seen.remove(obj_id)
 
                     return result
 
diff --git a/tests/unit/test_serializer.py b/tests/unit/test_serializer.py
index b26990b12..362997048 100644
--- a/tests/unit/test_serializer.py
+++ b/tests/unit/test_serializer.py
@@ -368,6 +368,29 @@ def __init__(self):
     assert json.loads(serializer.encode(obj)) == {"field": "value"}
 
 
+def test_slots_single_string():
+    class SingleSlotClass:
+        __slots__ = "query"
+
+        def __init__(self):
+            self.query = "SELECT 1"
+
+    obj = SingleSlotClass()
+    serializer = EventSerializer()
+    assert json.loads(serializer.encode(obj)) == {"query": "SELECT 1"}
+
+
+def test_slots_circular_reference():
+    class SlotNode:
+        __slots__ = ("next",)
+
+    node = SlotNode()
+    node.next = node
+
+    serializer = EventSerializer()
+    assert json.loads(serializer.encode(node)) == {"next": "SlotNode"}
+
+
 def test_deeply_nested_object_does_not_hang():
     class Inner:
         def __init__(self):