diff --git a/04/src/main.rs b/04/src/main.rs
index e776414..6be5fe3 100644
--- a/04/src/main.rs
+++ b/04/src/main.rs
@@ -49,6 +49,8 @@ fn part_b(cards: &[Card]) {
         result += additional_counts;
     }
 
+    assert_eq!(register.len(), 0);
+
     println!("{result}");
 }