diff --git a/flashbax/buffers/prioritised_trajectory_buffer.py b/flashbax/buffers/prioritised_trajectory_buffer.py
index d93d6b1..2337bf8 100644
--- a/flashbax/buffers/prioritised_trajectory_buffer.py
+++ b/flashbax/buffers/prioritised_trajectory_buffer.py
@@ -798,6 +798,7 @@ def make_prioritised_trajectory_buffer(
     if max_size is not None:
         max_length_time_axis = max_size // add_batch_size
 
+    assert max_length_time_axis is not None
     init_fn = functools.partial(
         prioritised_init,
         add_batch_size=add_batch_size,
diff --git a/flashbax/buffers/trajectory_buffer.py b/flashbax/buffers/trajectory_buffer.py
index a06ad45..73b33b6 100644
--- a/flashbax/buffers/trajectory_buffer.py
+++ b/flashbax/buffers/trajectory_buffer.py
@@ -585,6 +585,7 @@ def make_trajectory_buffer(
     if max_size is not None:
         max_length_time_axis = max_size // add_batch_size
 
+    assert max_length_time_axis is not None
     init_fn = functools.partial(
         init,
         add_batch_size=add_batch_size,