diff --git a/flask/testing.py b/flask/testing.py
index dd879c37..3b2f26ef 100644
--- a/flask/testing.py
+++ b/flask/testing.py
@@ -35,16 +35,14 @@ def make_test_environ_builder(app, path='/', base_url=None, json=None, *args, **
             if url.query:
                 path += '?' + url.query
 
-    if json:
+    if json is not None:
         if 'data' in kwargs:
             raise RuntimeError('Client cannot provide both `json` and `data`')
         kwargs['data'] = json_dumps(json)
 
         # Only set Content-Type when not explicitly provided
-        if 'Content-Type' not in kwargs.get('headers', {}):
-            new_headers = kwargs.get('headers', {}).copy()
-            new_headers['Content-Type'] = 'application/json'
-            kwargs['headers'] = new_headers
+        if 'content_type' not in kwargs:
+            kwargs['content_type'] = 'application/json'
 
     return EnvironBuilder(path, base_url, *args, **kwargs)
 
diff --git a/tests/test_testing.py b/tests/test_testing.py
index b3f4defe..727f08f7 100644
--- a/tests/test_testing.py
+++ b/tests/test_testing.py
@@ -215,6 +215,7 @@ def test_json_request():
         json_data = {'drink': {'gin': 1, 'tonic': True}, 'price': 10}
         rv = c.post('/api', json=json_data)
         assert rv.status_code == 200
+        assert flask.request.is_json
         assert flask.request.get_json() == json_data
 
 def test_subdomain():