diff --git a/requests/sessions.py b/requests/sessions.py
index 0fad2543a0..45be9733e5 100644
--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -557,7 +557,7 @@ def send(self, request, **kwargs):
 
         # It's possible that users might accidentally send a Request object.
         # Guard against that specific failure case.
-        if not isinstance(request, PreparedRequest):
+        if isinstance(request, Request):
             raise ValueError('You can only send PreparedRequests.')
 
         # Set up variables needed for resolve_redirects and dispatching of hooks
diff --git a/tests/test_requests.py b/tests/test_requests.py
index ebef0ba37d..2614667442 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -638,6 +638,14 @@ def test_unicode_method_name_with_request_object(self, httpbin):
         resp = s.send(prep)
         assert resp.status_code == 200
 
+    def test_non_prepared_request_error(self):
+        s = requests.Session()
+        req = requests.Request(u('POST'), '/')
+
+        with pytest.raises(ValueError) as e:
+            s.send(req)
+        assert str(e.value) == 'You can only send PreparedRequests.'
+
     def test_custom_content_type(self, httpbin):
         r = requests.post(
             httpbin('post'),