diff --git a/test_isort.py b/test_isort.py
index <HASH>..<HASH> 100644
--- a/test_isort.py
+++ b/test_isort.py
@@ -673,3 +673,6 @@ def test_settings_combine_instead_of_overwrite():
     """Test to ensure settings combine logically, instead of fully overwriting."""
     assert set(SortImports(known_standard_library=['not_std_library']).config['known_standard_library']) == \
            set(SortImports().config['known_standard_library'] + ['not_std_library'])
+
+    assert set(SortImports(not_known_standard_library=['thread']).config['known_standard_library']) == \
+           set(item for item in SortImports().config['known_standard_library'] if item != 'thread')
Add test to ensure full configurability is still present using 'not_' settings, since settings are no longer fully replaced each time