diff --git a/ThinkPythonNotebooks.zip b/ThinkPythonNotebooks.zip
index f5c7866..e4202d4 100644
Binary files a/ThinkPythonNotebooks.zip and b/ThinkPythonNotebooks.zip differ
diff --git a/chapters/chap13.ipynb b/chapters/chap13.ipynb
index ba90420..fe8f516 100644
--- a/chapters/chap13.ipynb
+++ b/chapters/chap13.ipynb
@@ -753,6 +753,18 @@
     "db"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "47685eb7",
+   "metadata": {
+    "tags": []
+   },
+   "source": [
+    "If you get an error like `db type could not be determined`, the most likely cause is that a file with the same name already exists but is not a valid shelve database (for example, it may be corrupted or created by something else).\n",
+    "\n",
+    "In that case, the simplest solution is to delete the existing file and run the code again so that `shelve.open` can create a new database."
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "0e4a2fb3",