diff --git a/gongish/response_formatters.py b/gongish/response_formatters.py
index d289051..3237fdc 100644
--- a/gongish/response_formatters.py
+++ b/gongish/response_formatters.py
@@ -1,8 +1,19 @@
 try:
     import ujson as jsonlib
+
+    def dump_json(v, indent):
+        # ujson 4.x patch
+        # https://github.com/ultrajson/ultrajson/issues/317
+        if indent is None:
+            indent = 0
+        return jsonlib.dumps(v, indent=indent)
+
+
 except ImportError:  # pragma: no cover
     import json as jsonlib
 
+    dump_json = jsonlib.dumps
+
 
 class ResponseFormattersMixin:
     @staticmethod
@@ -16,7 +27,7 @@ def format_json(request, response, indent=None):
         # Format in JSON
         response.type = "application/json"
         response.charset = "utf-8"
-        response.body = jsonlib.dumps(response.body, indent=indent)
+        response.body = dump_json(response.body, indent=indent)
 
     @staticmethod
     def format_binary(request, response):
diff --git a/tests/test_response_formatter.py b/tests/test_response_formatter.py
index 89419c8..4e22362 100644
--- a/tests/test_response_formatter.py
+++ b/tests/test_response_formatter.py
@@ -13,11 +13,33 @@ class MyApp(Application):
     def get():
         return dict(a=1)
 
+    @app.route("/none")
+    def get():
+        return
+
+    @app.route("/int")
+    def get():
+        return 12
+
+    @app.route("/str")
+    def get():
+        return 'thestr'
+
     testapp = webtest.TestApp(app)
+
     resp = testapp.get("/")
     assert resp.json == dict(a=1)
     assert resp.status == "200 OK"
 
+    resp = testapp.get("/none")
+    assert resp.json is None
+
+    resp = testapp.get("/int")
+    assert resp.json == 12
+
+    resp = testapp.get("/str")
+    assert resp.json == 'thestr'
+
 
 def test_text_formatter():
     class MyApp(Application):