diff --git a/swh/web/api/apiresponse.py b/swh/web/api/apiresponse.py
--- a/swh/web/api/apiresponse.py
+++ b/swh/web/api/apiresponse.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2017-2019  The Software Heritage developers
+# Copyright (C) 2017-2021  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU Affero General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -136,10 +136,12 @@
     # get request status code
     doc_data["status_code"] = options.get("status", 200)
 
+    accepted_media_type = getattr(request, "accepted_media_type", "application/json")
+
     # when requesting HTML, typically when browsing the API through its
     # documented views, we need to enrich the input data with documentation
     # and render the apidoc HTML template
-    if request.accepted_media_type == "text/html":
+    if accepted_media_type == "text/html":
         doc_data["response_data"] = data
         if data is not None:
             doc_data["response_data"] = json.dumps(
@@ -166,7 +168,7 @@
         response = Response(
             data,
             headers=headers,
-            content_type=request.accepted_media_type,
+            content_type=accepted_media_type,
             status=doc_data["status_code"],
         )
 
@@ -209,7 +211,7 @@
         "reason": str(exception),
     }
 
-    if request.accepted_media_type == "text/html":
+    if getattr(request, "accepted_media_type", None) == "text/html":
         error_data["reason"] = escape(error_data["reason"])
 
     if get_config()["debug"]:
diff --git a/swh/web/api/views/graph.py b/swh/web/api/views/graph.py
--- a/swh/web/api/views/graph.py
+++ b/swh/web/api/views/graph.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2020  The Software Heritage developers
+# Copyright (C) 2020-2021  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU Affero General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -11,6 +11,7 @@
 
 from django.http.response import StreamingHttpResponse
 from rest_framework.decorators import renderer_classes
+from rest_framework.renderers import JSONRenderer
 from rest_framework.request import Request
 from rest_framework.response import Response
 
@@ -123,7 +124,7 @@
 
 
 @api_route(r"/graph/(?P<graph_query>.+)/", "api-1-graph")
-@renderer_classes([PlainTextRenderer])
+@renderer_classes([JSONRenderer, PlainTextRenderer])
 def api_graph_proxy(
     request: Request, graph_query: str
 ) -> Union[Response, StreamingHttpResponse]:
@@ -142,7 +143,7 @@
     # graph stats and counter endpoint responses are not streamed
     if response.headers.get("Transfer-Encoding") != "chunked":
         return Response(
-            response.text,
+            response.json(),
             status=response.status_code,
             content_type=response.headers["Content-Type"],
         )
diff --git a/swh/web/tests/api/views/test_graph.py b/swh/web/tests/api/views/test_graph.py
--- a/swh/web/tests/api/views/test_graph.py
+++ b/swh/web/tests/api/views/test_graph.py
@@ -4,7 +4,6 @@
 # See top-level LICENSE file for more information
 
 import hashlib
-import json
 import textwrap
 
 from django.http.response import StreamingHttpResponse
@@ -126,7 +125,7 @@
 
     resp = check_http_get_response(api_client, url, status_code=200)
     assert resp.content_type == "application/json"
-    assert resp.content == json.dumps(_response_json).encode()
+    assert resp.data == _response_json
 
 
 def test_graph_ndjson_response(api_client, keycloak_oidc, requests_mock):
@@ -254,4 +253,18 @@
 
     resp = check_http_get_response(api_client, url, status_code=200)
     assert resp.content_type == "application/json"
-    assert resp.content == json.dumps(_response_json).encode()
+    assert resp.data == _response_json
+
+
+def test_graph_response_invalid_accept_header(api_client):
+    url = reverse(
+        "api-1-graph",
+        url_args={"graph_query": "stats"},
+        query_params={"resolve_origins": "true"},
+    )
+
+    resp = api_client.get(url, HTTP_ACCEPT="text/html")
+    assert resp.status_code == 406
+    assert resp.content_type == "application/json"
+    assert resp.data["exception"] == "NotAcceptable"
+    assert resp.data["reason"] == "Could not satisfy the request Accept header."