diff --git a/scrapy/settings/__init__.py b/scrapy/settings/__init__.py
index bbe8ef481..938b93564 100644
--- a/scrapy/settings/__init__.py
+++ b/scrapy/settings/__init__.py
@@ -88,7 +88,7 @@ class Settings(object):
         return dict(value)
 
     def set(self, name, value, priority='project'):
-        assert not self.frozen, "Trying to modify an immutable Settings object"
+        self._assert_mutability()
         if isinstance(priority, six.string_types):
             priority = SETTINGS_PRIORITIES[priority]
         if name not in self.attributes:
@@ -97,18 +97,22 @@ class Settings(object):
             self.attributes[name].set(value, priority)
 
     def setdict(self, values, priority='project'):
-        assert not self.frozen, "Trying to modify an immutable Settings object"
+        self._assert_mutability()
         for name, value in six.iteritems(values):
             self.set(name, value, priority)
 
     def setmodule(self, module, priority='project'):
-        assert not self.frozen, "Trying to modify an immutable Settings object"
+        self._assert_mutability()
         if isinstance(module, six.string_types):
             module = import_module(module)
         for key in dir(module):
             if key.isupper():
                 self.set(key, getattr(module, key), priority)
 
+    def _assert_mutability(self):
+        if self.frozen:
+            raise TypeError("Trying to modify an immutable Settings object")
+
     def copy(self):
         return copy.deepcopy(self)
 
diff --git a/tests/test_settings/__init__.py b/tests/test_settings/__init__.py
index c7e0914d6..38797ad45 100644
--- a/tests/test_settings/__init__.py
+++ b/tests/test_settings/__init__.py
@@ -213,7 +213,7 @@ class SettingsTest(unittest.TestCase):
 
     def test_freeze(self):
         self.settings.freeze()
-        with self.assertRaises(AssertionError) as cm:
+        with self.assertRaises(TypeError) as cm:
             self.settings.set('TEST_BOOL', False)
             self.assertEqual(str(cm.exception),
                              "Trying to modify an immutable Settings object")