diff --git a/day07/day07.py b/day07/day07.py
index 199c6e2..84b5f31 100755
--- a/day07/day07.py
+++ b/day07/day07.py
@@ -4,6 +4,7 @@ from typing import Sequence
 
 
 def rangesum(rn: range):
+    assert rn.step == 1, "this function doesn't work when range step != 1"
     return (((rn.stop + 1) * rn.stop) // 2) - (((rn.start + 1) * rn.start) // 2)