diff --git a/day15/common.py b/day15/common.py
index 168c724..e8d2dcd 100644
--- a/day15/common.py
+++ b/day15/common.py
@@ -2,7 +2,7 @@ from functools import reduce
 
 
 def yield_numbers(line: str, target: int):
-    nums = {}
+    nums = [None]*target
     turn = 0
     last_spoken = 0
     for starting in map(int, line.split(",")):
@@ -11,7 +11,7 @@ def yield_numbers(line: str, target: int):
         last_spoken = starting
         yield turn, last_spoken, nums
     for t in range(turn, target):
-        n = t - nums[last_spoken] if last_spoken in nums else 0
+        n = t - nums[last_spoken] if nums[last_spoken] is not None else 0
         nums[last_spoken] = t
         last_spoken = n
         yield t+1, last_spoken, nums