diff --git a/ipynb/Cheryl-and-Eve.ipynb b/ipynb/Cheryl-and-Eve.ipynb
index 1d82f5f..701d513 100644
--- a/ipynb/Cheryl-and-Eve.ipynb
+++ b/ipynb/Cheryl-and-Eve.ipynb
@@ -328,7 +328,7 @@
     "def good_dates(puzzle, n=10) -> BeliefState:\n",
     "    \"\"\"Pick a set of `n` dates for which the `puzzle` has a unique solution.\"\"\"\n",
     "    while True:\n",
-    "        random_dates = set(random.sample(many_dates, n))\n",
+    "        random_dates = set(random.sample(tuple(many_dates), n))\n",
     "        if know(puzzle(random_dates)):\n",
     "            return random_dates"
    ]