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
@@ -5,7 +5,7 @@
 
 import base64
 import datetime
-from json import JSONDecoder, JSONEncoder
+import json
 import types
 from uuid import UUID
 
@@ -28,7 +28,7 @@
     if content_type.startswith('application/x-msgpack'):
         r = msgpack_loads(response.content)
     elif content_type.startswith('application/json'):
-        r = response.json(cls=SWHJSONDecoder)
+        r = json.loads(response.text, cls=SWHJSONDecoder)
     else:
         raise ValueError('Wrong content type `%s` for API response'
                          % content_type)
@@ -36,7 +36,7 @@
     return r
 
 
-class SWHJSONEncoder(JSONEncoder):
+class SWHJSONEncoder(json.JSONEncoder):
     """JSON encoder for data structures generated by Software Heritage.
 
     This JSON encoder extends the default Python JSON encoder and adds
@@ -101,7 +101,7 @@
                 return list(iterable)
 
 
-class SWHJSONDecoder(JSONDecoder):
+class SWHJSONDecoder(json.JSONDecoder):
     """JSON decoder for data structures encoded with SWHJSONEncoder.
 
     This JSON decoder extends the default Python JSON decoder,
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
@@ -9,12 +9,15 @@
 from uuid import UUID
 
 import arrow
+import requests
+import requests_mock
 
 from swh.core.api.serializers import (
     SWHJSONDecoder,
     SWHJSONEncoder,
     msgpack_dumps,
-    msgpack_loads
+    msgpack_loads,
+    decode_response
 )
 
 
@@ -79,3 +82,11 @@
     def test_generator_msgpack(self):
         data = msgpack_dumps(self.generator)
         self.assertEqual(self.gen_lst, msgpack_loads(data))
+
+    @requests_mock.Mocker()
+    def test_decode_response_json(self, mock_requests):
+        mock_requests.get('https://example.org/test/data',
+                          json=self.encoded_data,
+                          headers={'content-type': 'application/json'})
+        response = requests.get('https://example.org/test/data')
+        assert decode_response(response) == self.data