diff --git a/mem0/vector_stores/pgvector.py b/mem0/vector_stores/pgvector.py
index 454313886..f55435572 100644
--- a/mem0/vector_stores/pgvector.py
+++ b/mem0/vector_stores/pgvector.py
@@ -89,6 +89,10 @@ def _build_filter_conditions(filters):
                     raise ValueError(f"Unsupported filter operator: {op}")
                 template, is_numeric = OPERATOR_SQL_MAP[op]
                 if op in ("in", "nin"):
+                    if not isinstance(op_value, list):
+                        raise ValueError(
+                            f"Filter operator {op!r} for key {key!r} requires a list value, got {type(op_value).__name__}"
+                        )
                     str_list = [str(v) for v in op_value]
                     conditions.append(template)
                     params.extend([key, str_list])
diff --git a/tests/vector_stores/test_pgvector.py b/tests/vector_stores/test_pgvector.py
index f4a3351af..308246f1b 100644
--- a/tests/vector_stores/test_pgvector.py
+++ b/tests/vector_stores/test_pgvector.py
@@ -2508,3 +2508,20 @@ class TestBuildFilterConditions(unittest.TestCase):
     def test_numeric_scalar_becomes_string(self):
         conditions, params = _build_filter_conditions({"priority": 42})
         self.assertEqual(params, ["priority", "42"])
+
+    def test_in_rejects_string_value(self):
+        """Passing a string to 'in' would iterate characters and produce a misleading ANY() clause."""
+        with self.assertRaises(ValueError, msg="Expected ValueError for non-list 'in' value"):
+            _build_filter_conditions({"user_id": {"in": "alice"}})
+
+    def test_in_rejects_dict_value(self):
+        with self.assertRaises(ValueError):
+            _build_filter_conditions({"user_id": {"in": {"$gt": 0}}})
+
+    def test_nin_rejects_string_value(self):
+        with self.assertRaises(ValueError):
+            _build_filter_conditions({"user_id": {"nin": "alice"}})
+
+    def test_in_accepts_list_value(self):
+        conditions, params = _build_filter_conditions({"user_id": {"in": ["alice", "bob"]}})
+        self.assertEqual(params, ["user_id", ["alice", "bob"]])