diff --git a/bertopic/_bertopic.py b/bertopic/_bertopic.py
index cfafb58a..c78ffda2 100644
--- a/bertopic/_bertopic.py
+++ b/bertopic/_bertopic.py
@@ -5006,12 +5006,18 @@ def add_mappings(self, mappings: Mapping[int, int], topic_model: BERTopic):
     def add_new_topics(self, mappings: Mapping[int, int]):
         """Add new row(s) of topic mappings.
 
+        New topics did not exist at earlier states, so the intermediate
+        history columns are backfilled with the topic's own ``key`` to
+        keep ``mappings_`` a homogeneous integer matrix. Without this,
+        ``None`` placeholders break ``model.save(serialization="safetensors")``
+        which casts the matrix to ``np.array(..., dtype=int)``.
+
         Arguments:
             mappings: The mappings to add
         """
         length = len(self.mappings_[0])
         for key, value in mappings.items():
-            to_append = [key] + ([None] * (length - 2)) + [value]
+            to_append = [key] * (length - 1) + [value]
             self.mappings_.append(to_append)
 
 
diff --git a/tests/test_bertopic.py b/tests/test_bertopic.py
index 0d315f89..68b9e36c 100644
--- a/tests/test_bertopic.py
+++ b/tests/test_bertopic.py
@@ -1,6 +1,8 @@
 import copy
 import pytest
+import numpy as np
 from bertopic import BERTopic
+from bertopic._bertopic import TopicMapper
 import importlib.util
 
 
@@ -153,3 +155,34 @@ def test_full_model(model, documents, request):
     merged_model = BERTopic.merge_models([topic_model, topic_model1])
 
     assert len(merged_model.get_topic_info()) > len(topic_model.get_topic_info())
+
+
+def test_topic_mapper_add_new_topics_keeps_integer_matrix():
+    """Regression test for #2432: ``TopicMapper.add_new_topics`` must keep
+    ``mappings_`` as a homogeneous integer matrix.
+
+    Previously, ``add_new_topics`` inserted ``None`` placeholders for the
+    intermediate history columns, which broke ``model.save()`` because
+    ``_save_utils.save_topics`` casts the matrix to ``np.array(..., dtype=int)``.
+    """
+    mapper = TopicMapper(topics=[-1, 0, 1, 2])
+    # Simulate two prior reduce_topics calls so the matrix has more than 2
+    # columns (the buggy ``length - 2`` path is hidden when ``__init__``'s
+    # default 2-column shape is used).
+    for row in mapper.mappings_:
+        row.append(row[-1])
+        row.append(row[-1])
+    pre_existing = [list(row) for row in mapper.mappings_]
+
+    # New clusters discovered during partial_fit
+    mapper.add_new_topics({3: 2, 4: 3})
+
+    # The matrix must round-trip through ``np.array(..., dtype=int)``
+    # (mirrors what ``_save_utils.save_topics`` does).
+    matrix = np.array(mapper.mappings_, dtype=int)
+    # Pre-existing rows must be untouched.
+    assert mapper.mappings_[: len(pre_existing)] == pre_existing
+    # Original and current state of new rows must be preserved, and the
+    # intermediate history columns must be backfilled with the topic's own key.
+    assert (matrix[-2, :-1] == 3).all() and matrix[-2, -1] == 2
+    assert (matrix[-1, :-1] == 4).all() and matrix[-1, -1] == 3