diff --git a/scrapy/utils/reqser.py b/scrapy/utils/reqser.py
index 5ea2aafb8..503d7b133 100644
--- a/scrapy/utils/reqser.py
+++ b/scrapy/utils/reqser.py
@@ -71,25 +71,20 @@ def request_from_dict(d, spider=None):
 
 
 def _find_method(obj, func):
-    if obj:
-        try:
-            func_self = func.__self__
-        except AttributeError:  # func has no __self__
-            pass
-        else:
-            if func_self is obj:
-                members = inspect.getmembers(obj, predicate=inspect.ismethod)
-                for name, obj_func in members:
-                    # We need to use __func__ to access the original
-                    # function object because instance method objects
-                    # are generated each time attribute is retrieved from
-                    # instance.
-                    #
-                    # Reference: The standard type hierarchy
-                    # https://docs.python.org/3/reference/datamodel.html
-                    if obj_func.__func__ is func.__func__:
-                        return name
-    raise ValueError("Function %s is not a method of: %s" % (func, obj))
+    # Only instance methods contain ``__func__``
+    if obj and hasattr(func, '__func__'):
+        members = inspect.getmembers(obj, predicate=inspect.ismethod)
+        for name, obj_func in members:
+            # We need to use __func__ to access the original
+            # function object because instance method objects
+            # are generated each time attribute is retrieved from
+            # instance.
+            #
+            # Reference: The standard type hierarchy
+            # https://docs.python.org/3/reference/datamodel.html
+            if obj_func.__func__ is func.__func__:
+                return name
+    raise ValueError("Function %s is not an instance method in: %s" % (func, obj))
 
 
 def _get_method(obj, name):
diff --git a/tests/test_utils_reqser.py b/tests/test_utils_reqser.py
index de94ec960..ee68cf6b1 100644
--- a/tests/test_utils_reqser.py
+++ b/tests/test_utils_reqser.py
@@ -102,6 +102,12 @@ class RequestSerializationTest(unittest.TestCase):
                     errback=self.spider.handle_error)
         self._assert_serializes_ok(r, spider=self.spider)
 
+    def test_delegated_callback_serialization(self):
+        r = Request("http://www.example.com",
+                    callback=self.spider.delegated_callback,
+                    errback=self.spider.handle_error)
+        self._assert_serializes_ok(r, spider=self.spider)
+
     def test_unserializable_callback1(self):
         r = Request("http://www.example.com", callback=lambda x: x)
         self.assertRaises(ValueError, request_to_dict, r)
@@ -132,6 +138,11 @@ class TestSpiderMixin:
         pass
 
 
+class TestSpiderDelegation:
+    def delegated_callback(self, response):
+        pass
+
+
 def parse_item(response):
     pass
 
@@ -155,6 +166,9 @@ class TestSpider(Spider, TestSpiderMixin):
     __parse_item_reference = private_parse_item
     __handle_error_reference = private_handle_error
 
+    def __init__(self):
+        self.delegated_callback = TestSpiderDelegation().delegated_callback
+
     def parse_item(self, response):
         pass