diff --git a/cassandra/util.py b/cassandra/util.py
index 593c264033..e420a382b0 100644
--- a/cassandra/util.py
+++ b/cassandra/util.py
@@ -787,7 +787,13 @@ def _insert_unchecked(self, key, flat_key, value):
         self._index[flat_key] = len(self._items) - 1
 
     def _serialize_key(self, key):
-        return self.cass_key_type.serialize(key, self.protocol_version)
+        try:
+            return self.cass_key_type.serialize(key, self.protocol_version)
+        except Exception:
+            # A key that cannot be serialized with the map's key type cannot
+            # be present, so treat it as missing to keep Mapping semantics
+            # (get() returns the default, `in` returns False).
+            raise KeyError(str(key)) from None
 
 
 @total_ordering
diff --git a/tests/unit/test_orderedmap.py b/tests/unit/test_orderedmap.py
index 156bbd5f30..85d5aed6fa 100644
--- a/tests/unit/test_orderedmap.py
+++ b/tests/unit/test_orderedmap.py
@@ -184,3 +184,16 @@ def test_normalized_lookup(self):
         assert om[{'one': 1}] is om[{u'one': 1}]
         assert om[{'two': 2}] is om[{u'two': 2}]
         assert om[{'one': 1}] is not om[{'two': 2}]
+
+    def test_unserializable_key_treated_as_missing(self):
+        # a key that cannot be serialized with the map's key type cannot be
+        # present, so lookups behave like a plain dict instead of leaking the
+        # serializer's exception
+        om = OrderedMapSerializedKey(UTF8Type, 3)
+        om._insert_unchecked('one', UTF8Type.serialize('one', 3), 1)
+
+        assert om.get(None) is None
+        assert om.get(None, 2) == 2
+        assert None not in om
+        with pytest.raises(KeyError):
+            om[None]