diff --git a/flake8/options/manager.py b/flake8/options/manager.py
index da3f73e..9faaf7c 100644
--- a/flake8/options/manager.py
+++ b/flake8/options/manager.py
@@ -190,6 +190,7 @@ class OptionManager(object):
             List of strings that are the error/warning codes with which to
             extend the default ignore list.
         """
+        LOG.debug('Extending default ignore list with %r', error_codes)
         self.extended_default_ignore.update(error_codes)
 
     def generate_versions(self, format_str='%(name)s: %(version)s'):
diff --git a/tests/unit/test_option_manager.py b/tests/unit/test_option_manager.py
index 660b1a6..99c5d18 100644
--- a/tests/unit/test_option_manager.py
+++ b/tests/unit/test_option_manager.py
@@ -184,3 +184,13 @@ def test_generate_epilog(optmanager):
 
     optmanager.generate_epilog()
     assert optmanager.parser.epilog == expected_value
+
+
+def test_extend_default_ignore(optmanager):
+    """Verify that we update the extended default ignore list."""
+    assert optmanager.extended_default_ignore == set()
+
+    optmanager.extend_default_ignore(['T100', 'T101', 'T102'])
+    assert optmanager.extended_default_ignore == set(['T100',
+                                                      'T101',
+                                                      'T102'])