diff --git a/cooperative_cuisine/scores.py b/cooperative_cuisine/scores.py
index 03c9bfd669a367dc68adf8286aeef61ec608429e..05891b2f4e950f1ceb1895a7ab03712d677feae6 100644
--- a/cooperative_cuisine/scores.py
+++ b/cooperative_cuisine/scores.py
@@ -120,10 +120,10 @@ def stepped_score(
         raise ValueError("steps and vals must have the same length")
 
     for threshold, ratio in zip(reversed(steps), reversed(score_ratios)):
-        if time_percentage >= threshold:
+        if max(time_percentage, 0) >= threshold:
             return float(np.round(max_score * ratio, decimals=round_decimals))
 
-    assert False, "Should not reach here."
+    assert False, "This should not happen."
 
 
 class ScoreViaHooks(HookCallbackClass):