diff --git a/ch02/05_bpe-from-scratch/bpe-from-scratch.ipynb b/ch02/05_bpe-from-scratch/bpe-from-scratch.ipynb
index 16ce548..859cc78 100644
--- a/ch02/05_bpe-from-scratch/bpe-from-scratch.ipynb
+++ b/ch02/05_bpe-from-scratch/bpe-from-scratch.ipynb
@@ -629,7 +629,7 @@
     "        \"\"\"\n",
     "        # Save vocabulary\n",
     "        with open(vocab_path, \"w\", encoding=\"utf-8\") as file:\n",
-    "            json.dump({k: v for k, v in self.vocab.items()}, file, ensure_ascii=False, indent=2)\n",
+    "            json.dump(self.vocab, file, ensure_ascii=False, indent=2)\n",
     "\n",
     "        # Save BPE merges as a list of dictionaries\n",
     "        with open(bpe_merges_path, \"w\", encoding=\"utf-8\") as file:\n",
@@ -667,6 +667,9 @@
     "    def find_freq_pair(token_ids, mode=\"most\"):\n",
     "        pairs = Counter(zip(token_ids, token_ids[1:]))\n",
     "\n",
+    "        if not pairs:\n",
+    "            return None\n",
+    "\n",
     "        if mode == \"most\":\n",
     "            return max(pairs.items(), key=lambda x: x[1])[0]\n",
     "        elif mode == \"least\":\n",