msgpack
diff --git a/msgpack/_packer.pyx b/msgpack/_packer.pyx
@@ -285,6 +285,8 @@ cdef class Packer(object):
                 o = self._default(o)
                 default_used = 1
                 continue
+            elif self.datetime and PyDateTime_CheckExact(o):
+                PyErr_Format(ValueError, b"can not serialize '%.200s' object where tzinfo=None", Py_TYPE(o).tp_name)
             else:
                 PyErr_Format(TypeError, b"can not serialize '%.200s' object", Py_TYPE(o).tp_name)
             return ret

diff --git a/msgpack/fallback.py b/msgpack/fallback.py
@@ -874,6 +874,10 @@ def _pack(
                 obj = self._default(obj)
                 default_used = 1
                 continue
+
+            if self._datetime and check(obj, _DateTime):
+                raise ValueError("Cannot serialize %r where tzinfo=None" % (obj,))
+
             raise TypeError("Cannot serialize %r" % (obj,))
 
     def pack(self, obj):

diff --git a/test/test_timestamp.py b/test/test_timestamp.py
@@ -140,3 +140,19 @@ def test_issue451():
 
     unpacked = msgpack.unpackb(packed, timestamp=3)
     assert dt == unpacked
+
+
+@pytest.mark.skipif(sys.version_info[0] == 2, reason="datetime support is PY3+ only")
+def test_pack_datetime_without_tzinfo():
+    dt = datetime.datetime(1970, 1, 1, 0, 0, 42, 14)
+    with pytest.raises(ValueError, match="where tzinfo=None"):
+        packed = msgpack.packb(dt, datetime=True)
+
+    dt = datetime.datetime(1970, 1, 1, 0, 0, 42, 14)
+    packed = msgpack.packb(dt, datetime=True, default=lambda x: None)
+    assert packed == msgpack.packb(None)
+
+    dt = datetime.datetime(1970, 1, 1, 0, 0, 42, 14, tzinfo=_utc)
+    packed = msgpack.packb(dt, datetime=True)
+    unpacked = msgpack.unpackb(packed, timestamp=3)
+    assert unpacked == dt