diff --git a/src/MEDS_tabular_automl/generate_summarized_reps.py b/src/MEDS_tabular_automl/generate_summarized_reps.py
index 9e9202c..9888aa3 100644
--- a/src/MEDS_tabular_automl/generate_summarized_reps.py
+++ b/src/MEDS_tabular_automl/generate_summarized_reps.py
@@ -18,7 +18,8 @@
 
 
 def time_aggd_col_alias_fntr(window_size: str, agg: str) -> Callable[[str], str]:
-    assert agg is not None, "agg must be provided"
+    if agg is None:
+        raise ValueError("Aggregation type 'agg' must be provided")
 
     def f(c: str) -> str:
         return "/".join([window_size] + c.split("/") + [agg])