diff --git a/lessons/13_Step_10.ipynb b/lessons/13_Step_10.ipynb
index c5a9bb15..cb8ca8dc 100644
--- a/lessons/13_Step_10.ipynb
+++ b/lessons/13_Step_10.ipynb
@@ -132,7 +132,7 @@
     "pd = numpy.zeros((ny, nx))\n",
     "b  = numpy.zeros((ny, nx))\n",
     "x  = numpy.linspace(xmin, xmax, nx)\n",
-    "y  = numpy.linspace(xmin, xmax, ny)\n",
+    "y  = numpy.linspace(ymin, ymax, ny)\n",
     "\n",
     "# Source\n",
     "b[int(ny / 4), int(nx / 4)]  = 100\n",