diff --git a/lessons/03_preprocessing.ipynb b/lessons/03_preprocessing.ipynb
index 6465fb4..a59afdf 100644
--- a/lessons/03_preprocessing.ipynb
+++ b/lessons/03_preprocessing.ipynb
@@ -223,7 +223,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "data = data.dropna(subset='sex')\n",
+    "data = data.dropna(subset=['sex'])\n",
     "\n",
     "# Now this line will return an empty dataframe\n",
     "data[data['sex'].isna()]"
@@ -431,7 +431,7 @@
    "source": [
     "data = pd.read_csv('../data/penguins.csv')\n",
     "data.replace('.', np.nan, inplace=True)\n",
-    "data = data.dropna(subset='sex')\n"
+    "data = data.dropna(subset=['sex'])\n"
    ]
   },
   {