diff --git a/scrapy/http/request/__init__.py b/scrapy/http/request/__init__.py
index 1435d91de..b9c5f8541 100644
--- a/scrapy/http/request/__init__.py
+++ b/scrapy/http/request/__init__.py
@@ -27,6 +27,10 @@ class Request(object_ref):
         assert isinstance(priority, int), "Request priority not an integer: %r" % priority
         self.priority = priority
 
+        if callback is not None and not callable(callback):
+            raise TypeError('callback must be a function, got %s' % type(callback).__name__)
+        if errback is not None and not callable(errback):
+            raise TypeError('errback must be a function, got %s' % type(errback).__name__)
         assert callback or not errback, "Cannot use errback without a callback"
         self.callback = callback
         self.errback = errback
diff --git a/tests/test_http_request.py b/tests/test_http_request.py
index bbce537f4..9b0ee63dc 100644
--- a/tests/test_http_request.py
+++ b/tests/test_http_request.py
@@ -235,6 +235,26 @@ class RequestTest(unittest.TestCase):
         self.assertRaises(AttributeError, setattr, r, 'url', 'http://example2.com')
         self.assertRaises(AttributeError, setattr, r, 'body', 'xxx')
 
+    def test_callback_is_callable(self):
+        def a_function():
+            pass
+        r = self.request_class('http://example.com')
+        self.assertIsNone(r.callback)
+        r = self.request_class('http://example.com', a_function)
+        self.assertIs(r.callback, a_function)
+        with self.assertRaises(TypeError):
+            self.request_class('http://example.com', 'a_function')
+
+    def test_errback_is_callable(self):
+        def a_function():
+            pass
+        r = self.request_class('http://example.com')
+        self.assertIsNone(r.errback)
+        r = self.request_class('http://example.com', a_function, errback=a_function)
+        self.assertIs(r.errback, a_function)
+        with self.assertRaises(TypeError):
+            self.request_class('http://example.com', a_function, errback='a_function')
+
 
 class FormRequestTest(RequestTest):