diff --git a/swh/core/api/serializers.py b/swh/core/api/serializers.py
--- a/swh/core/api/serializers.py
+++ b/swh/core/api/serializers.py
@@ -208,8 +208,13 @@
         return obj
 
     try:
-        return msgpack.unpackb(data, raw=False,
-                               object_hook=decode_types)
+        try:
+            return msgpack.unpackb(data, raw=False,
+                                   object_hook=decode_types,
+                                   strict_map_key=False)
+        except TypeError:  # msgpack < 0.6.0
+            return msgpack.unpackb(data, raw=False,
+                                   object_hook=decode_types)
     except TypeError:  # msgpack < 0.5.2
         return msgpack.unpackb(data, encoding='utf-8',
                                object_hook=decode_types)
diff --git a/swh/core/api/tests/test_serializers.py b/swh/core/api/tests/test_serializers.py
--- a/swh/core/api/tests/test_serializers.py
+++ b/swh/core/api/tests/test_serializers.py
@@ -106,8 +106,12 @@
         self.assertEqual(self.encoded_data, json.loads(data))
 
     def test_round_trip_msgpack(self):
-        data = msgpack_dumps(self.data)
-        self.assertEqual(self.data, msgpack_loads(data))
+        original_data = {
+            **self.data,
+            'none_dict_key': {None: 42},
+        }
+        data = msgpack_dumps(original_data)
+        self.assertEqual(original_data, msgpack_loads(data))
 
     def test_round_trip_msgpack_extra_types(self):
         original_data = [ExtraType('baz', self.data), 'qux']