diff --git a/pyopencl/algorithm.py b/pyopencl/algorithm.py
index 0294c2e2e860a2fda212d1f36175f82719bc1970..a116534001abd556bd04eecd19f5c53cb772cb5c 100644
--- a/pyopencl/algorithm.py
+++ b/pyopencl/algorithm.py
@@ -728,7 +728,7 @@ def _get_arg_list(arg_list, prefix=""):
 class BuiltList:
     count: Optional[int]
     starts: Optional[pyopencl.array.Array]
-    lists: Optional[pyopencl.array.Array]
+    lists: Optional[pyopencl.array.Array] = None
     num_nonempty_lists: Optional[int] = None
     nonempty_indices: Optional[pyopencl.array.Array] = None
     compressed_indices: Optional[pyopencl.array.Array] = None