diff --git a/lessons/12_Step_9.ipynb b/lessons/12_Step_9.ipynb
index 8855c39b..e95d7f86 100644
--- a/lessons/12_Step_9.ipynb
+++ b/lessons/12_Step_9.ipynb
@@ -222,7 +222,7 @@
     "ny = 31\n",
     "c = 1\n",
     "dx = 2 / (nx - 1)\n",
-    "dy = 2 / (ny - 1)\n",
+    "dy = 1 / (ny - 1)\n",
     "\n",
     "\n",
     "##initial conditions\n",