diff --git a/tests/test_plugin.py b/tests/test_plugin.py
index 9ae955f..f722d3c 100644
--- a/tests/test_plugin.py
+++ b/tests/test_plugin.py
@@ -63,9 +63,6 @@ def easy_entries():
         ]),
     ]
 
-def hook_types(hooks, key):
-    return {type(hook) for hook in hooks[key]}
-
 def map_errors(errors):
     retval = {}
     for errkey in errors:
@@ -73,33 +70,20 @@ def map_errors(errors):
         retval.setdefault(key, set()).add(errid)
     return retval
 
-def test_registry_all_by_default():
-    hook_groups = HOOK_REGISTRY.group_by_directive()
-    hooks = hook_types(hook_groups, 'Transaction')
-    assert len(hooks) >= 2
-    assert TransactionError in hooks
-    assert PostingError in hooks
-
-def test_registry_one_exclude():
-    hook_groups = HOOK_REGISTRY.group_by_directive('-posting')
-    hooks = hook_types(hook_groups, 'Transaction')
-    assert len(hooks) >= 1
-    assert TransactionError in hooks
-    assert PostingError not in hooks
-
-def test_registry_exclude_then_include():
-    hook_groups = HOOK_REGISTRY.group_by_directive('-configured posting')
-    hooks = hook_types(hook_groups, 'Transaction')
-    assert len(hooks) >= 1
-    assert TransactionError not in hooks
-    assert PostingError in hooks
-
-def test_registry_include_then_exclude():
-    hook_groups = HOOK_REGISTRY.group_by_directive('configured -posting')
-    hooks = hook_types(hook_groups, 'Transaction')
-    assert len(hooks) >= 1
-    assert TransactionError in hooks
-    assert PostingError not in hooks
+@pytest.mark.parametrize('group_str,expected', [
+    (None, [TransactionError, PostingError]),
+    ('', [TransactionError, PostingError]),
+    ('all', [TransactionError, PostingError]),
+    ('Transaction', [TransactionError, PostingError]),
+    ('-posting', [TransactionError]),
+    ('-configured posting', [PostingError]),
+    ('configured -posting', [TransactionError]),
+])
+def test_registry_group_by_directive(group_str, expected):
+    args = () if group_str is None else (group_str,)
+    hook_groups = HOOK_REGISTRY.group_by_directive(*args)
+    actual = {type(hook) for hook in hook_groups['Transaction']}
+    assert actual.issuperset(expected)
 
 def test_registry_unknown_group_name():
     with pytest.raises(ValueError):