diff --git a/pandas/core/accessor.py b/pandas/core/accessor.py
index 06c4068f86bfe..c638b9e4ea117 100644
--- a/pandas/core/accessor.py
+++ b/pandas/core/accessor.py
@@ -148,6 +148,7 @@ def decorator(accessor):
                 UserWarning,
                 stacklevel=2)
         setattr(cls, name, CachedAccessor(name, accessor))
+        cls._accessors.add(name)
         return accessor
     return decorator
 
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 41d67c15c55b5..b67ed9cfd2241 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -326,6 +326,7 @@ def _constructor(self):
     _constructor_sliced = Series
     _deprecations = NDFrame._deprecations | frozenset(
         ['sortlevel', 'get_value', 'set_value', 'from_csv', 'from_items'])
+    _accessors = set()
 
     @property
     def _constructor_expanddim(self):
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 12bb09e8f8a8a..27668f925ce23 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -242,7 +242,8 @@ class Index(IndexOpsMixin, PandasObject):
 
     _engine_type = libindex.ObjectEngine
 
-    _accessors = frozenset(['str'])
+    _accessors = set(['str'])
+
     str = CachedAccessor("str", StringMethods)
 
     def __new__(cls, data=None, dtype=None, copy=False, name=None,
diff --git a/pandas/core/series.py b/pandas/core/series.py
index 2ed4c99b7a998..13e94f971d003 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -153,7 +153,7 @@ class Series(base.IndexOpsMixin, generic.NDFrame):
         Copy input data
     """
     _metadata = ['name']
-    _accessors = frozenset(['dt', 'cat', 'str'])
+    _accessors = set(['dt', 'cat', 'str'])
     _deprecations = generic.NDFrame._deprecations | frozenset(
         ['asobject', 'sortlevel', 'reshape', 'get_value', 'set_value',
          'from_csv', 'valid'])
diff --git a/pandas/tests/test_register_accessor.py b/pandas/tests/test_register_accessor.py
index fe0cf4c9b38af..33b9798b7606a 100644
--- a/pandas/tests/test_register_accessor.py
+++ b/pandas/tests/test_register_accessor.py
@@ -17,6 +17,7 @@ def ensure_removed(obj, attr):
             delattr(obj, attr)
         except AttributeError:
             pass
+        obj._accessors.discard(attr)
 
 
 class MyAccessor(object):
@@ -38,13 +39,14 @@ def method(self):
     (pd.DataFrame, pd.api.extensions.register_dataframe_accessor),
     (pd.Index, pd.api.extensions.register_index_accessor)
 ])
-def test_series_register(obj, registrar):
+def test_register(obj, registrar):
     with ensure_removed(obj, 'mine'):
         before = set(dir(obj))
         registrar('mine')(MyAccessor)
         assert obj([]).mine.prop == 'item'
         after = set(dir(obj))
         assert (before ^ after) == {'mine'}
+        assert 'mine' in obj._accessors
 
 
 def test_accessor_works():
diff --git a/scripts/validate_docstrings.py b/scripts/validate_docstrings.py
index 3863451757709..aed3eb2f1226d 100755
--- a/scripts/validate_docstrings.py
+++ b/scripts/validate_docstrings.py
@@ -182,7 +182,8 @@ def doc_parameters(self):
     @property
     def signature_parameters(self):
         if (inspect.isclass(self.method_obj)
-                and self.method_name.split('.')[-1] in {'dt', 'str', 'cat'}):
+                and self.method_name.split('.')[-1] in
+                self.method_obj._accessors):
             # accessor classes have a signature, but don't want to show this
             return tuple()
         try: