diff --git a/zerver/lib/request.py b/zerver/lib/request.py
index 0f18bb6b95..2037c0ab40 100644
--- a/zerver/lib/request.py
+++ b/zerver/lib/request.py
@@ -155,7 +155,7 @@ def has_request_variables(view_func):
                 try:
                     val = ujson.loads(val)
                 except Exception:
-                    raise JsonableError(_('argument "%s" is not valid json.') % (param.post_var_name,))
+                    raise JsonableError(_('Argument "%s" is not valid JSON.') % (param.post_var_name,))
 
                 error = param.validator(param.post_var_name, val)
                 if error:
diff --git a/zerver/tests/test_custom_profile_data.py b/zerver/tests/test_custom_profile_data.py
index 93836d8363..1d48470ff0 100644
--- a/zerver/tests/test_custom_profile_data.py
+++ b/zerver/tests/test_custom_profile_data.py
@@ -31,7 +31,7 @@ class CustomProfileFieldTest(ZulipTestCase):
         self.login(self.example_email("iago"))
         data = {"name": u"Phone", "field_type": "text id"}  # type: Dict[str, Any]
         result = self.client_post("/json/realm/profile_fields", info=data)
-        self.assert_json_error(result, u'argument "field_type" is not valid json.')
+        self.assert_json_error(result, u'Argument "field_type" is not valid JSON.')
 
         data["name"] = ""
         data["field_type"] = 100
diff --git a/zerver/tests/test_decorators.py b/zerver/tests/test_decorators.py
index ae49231fbc..572e21ba44 100644
--- a/zerver/tests/test_decorators.py
+++ b/zerver/tests/test_decorators.py
@@ -166,7 +166,7 @@ class DecoratorTestCase(TestCase):
         request.POST['numbers'] = 'bad_value'
         with self.assertRaises(JsonableError) as cm:
             get_total(request)
-        self.assertEqual(str(cm.exception), 'argument "numbers" is not valid json.')
+        self.assertEqual(str(cm.exception), 'Argument "numbers" is not valid JSON.')
 
         request.POST['numbers'] = ujson.dumps([1, 2, "what?", 4, 5, 6])
         with self.assertRaises(JsonableError) as cm:
diff --git a/zerver/tests/test_tutorial.py b/zerver/tests/test_tutorial.py
index 231a6b9b36..fe841ac39c 100644
--- a/zerver/tests/test_tutorial.py
+++ b/zerver/tests/test_tutorial.py
@@ -57,7 +57,7 @@ class TutorialTests(ZulipTestCase):
         self.assert_json_error(result, "Missing 'type' argument")
 
         result = self.client_post("/json/tutorial_send_message", raw_params)
-        self.assert_json_error(result, 'argument "type" is not valid json.')
+        self.assert_json_error(result, 'Argument "type" is not valid JSON.')
 
         raw_params = dict(
             type='INVALID',