diff --git a/pytypes/type_util.py b/pytypes/type_util.py
index f6227c8..976f96f 100644
--- a/pytypes/type_util.py
+++ b/pytypes/type_util.py
@@ -22,10 +22,10 @@ import typing
 import collections
 from inspect import isfunction, ismethod, isclass, ismodule
 try:
-    from backports.typing import Tuple, Dict, List, Set, Union, Any, \
+    from backports.typing import Tuple, Dict, List, Set, FrozenSet, Union, Any, \
         Sequence, Mapping, TypeVar, Container, Generic, Sized, Iterable
 except ImportError:
-    from typing import Tuple, Dict, List, Set, Union, Any, \
+    from typing import Tuple, Dict, List, Set, FrozenSet, Union, Any, \
         Sequence, Mapping, TypeVar, Container, Generic, Sized, Iterable
 try:
     # Python 3.7
@@ -465,9 +465,13 @@ def _deep_type(obj, checked, checked_len, depth = None, max_sample = None):
             tpl1 = tuple(_deep_type(t, checked, checked_len2, depth-1) for t in ksmpl)
             tpl2 = tuple(_deep_type(t, checked, checked_len2, depth-1) for t in vsmpl)
         res = Dict[Union[tpl1], Union[tpl2]]
-    elif res == set:
+    elif res == set or res == frozenset:
+        if res == set:
+            typ = Set
+        else:
+            typ = FrozenSet
         if len(obj) == 0:
-            return Empty[Set]
+            return Empty[typ]
         if max_sample == -1 or max_sample >= len(obj)-1 or len(obj) <= 2:
             tpl = tuple(_deep_type(t, checked, depth-1) for t in obj)
         else:
@@ -485,7 +489,7 @@ def _deep_type(obj, checked, checked_len, depth = None, max_sample = None):
                         j -= 1
                 smpl.append(next(itr))
             tpl = tuple(_deep_type(t, checked, depth-1) for t in smpl)
-        res = Set[Union[tpl]]
+        res = typ[Union[tpl]]
     elif res == types.GeneratorType:
         res = get_generator_type(obj)
     elif sys.version_info.major == 2 and isinstance(obj, types.InstanceType):

Frozenset seems to not be equal to set?
```python
>>> import typing
>>> from pytypes import type_util
>>> type_util._isinstance(frozenset({1, 2, 'a', None, 'b'}), typing.AbstractSet[typing.Union[str, int, None]])
False
>>> type_util._isinstance({1, 2, 'a', None, 'b'}, typing.AbstractSet[typing.Union[str, int, type(None)]])
True
>>> issubclass(frozenset, typing.AbstractSet)
True
```
diff --git a/tests/test_typechecker.py b/tests/test_typechecker.py
index ddd2745..9106d6b 100644
--- a/tests/test_typechecker.py
+++ b/tests/test_typechecker.py
@@ -4664,6 +4664,10 @@ class Test_utils(unittest.TestCase):
         # No exception.
         resolve_fw_decl(Foo)
 
+    # See: https://github.com/Stewori/pytypes/issues/35
+    def test_frozenset(self):
+        self.assertTrue(pytypes.is_of_type(frozenset({1, 2, 'a', None, 'b'}), typing.AbstractSet[typing.Union[str, int, None]]))
+
 
 if __name__ == '__main__':
     unittest.main()

pip install -e .[dev]
parse_log_pytest
pytest
pytest
apt-get update apt-get install -y gcc
3.6
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
attrs @ file:///opt/conda/conda-bld/attrs_1642510447205/work
certifi==2021.5.30
importlib-metadata @ file:///tmp/build/80754af9/importlib-metadata_1631916693255/work
iniconfig @ file:///home/linux1/recipes/ci/iniconfig_1610983019677/work
more-itertools @ file:///tmp/build/80754af9/more-itertools_1637733554872/work
packaging @ file:///tmp/build/80754af9/packaging_1637314298585/work
pluggy @ file:///tmp/build/80754af9/pluggy_1615976315926/work
py @ file:///opt/conda/conda-bld/py_1644396412707/work
pyparsing @ file:///tmp/build/80754af9/pyparsing_1635766073266/work
pytest==6.2.4
-e git+https://github.com/Stewori/pytypes.git@152a2218bfa1b96ae5d866542ee4ad148b1b7b5d#egg=pytypes
toml @ file:///tmp/build/80754af9/toml_1616166611790/work
typing_extensions @ file:///opt/conda/conda-bld/typing_extensions_1647553014482/work
zipp @ file:///tmp/build/80754af9/zipp_1633618647012/work

name: pytypes
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - attrs=21.4.0=pyhd3eb1b0_0
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2021.5.30=py36h06a4308_0
  - importlib-metadata=4.8.1=py36h06a4308_0
  - importlib_metadata=4.8.1=hd3eb1b0_0
  - iniconfig=1.1.1=pyhd3eb1b0_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.3=he6710b0_2
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - more-itertools=8.12.0=pyhd3eb1b0_0
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - packaging=21.3=pyhd3eb1b0_0
  - pip=21.2.2=py36h06a4308_0
  - pluggy=0.13.1=py36h06a4308_0
  - py=1.11.0=pyhd3eb1b0_0
  - pyparsing=3.0.4=pyhd3eb1b0_0
  - pytest=6.2.4=py36h06a4308_2
  - python=3.6.13=h12debd9_1
  - readline=8.2=h5eee18b_0
  - setuptools=58.0.4=py36h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - toml=0.10.2=pyhd3eb1b0_0
  - typing_extensions=4.1.1=pyh06a4308_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.6.4=h5eee18b_1
  - zipp=3.6.0=pyhd3eb1b0_0
  - zlib=1.2.13=h5eee18b_1
prefix: /opt/conda/envs/pytypes

tests/test_typechecker.py::Test_utils::test_frozenset
tests/test_typechecker.py::testfunc tests/test_typechecker.py::testfunc_err tests/test_typechecker.py::testfunc2 tests/test_typechecker.py::testfunc4 tests/test_typechecker.py::testfunc_None_ret tests/test_typechecker.py::testfunc_None_ret_err tests/test_typechecker.py::testfunc_None_arg tests/test_typechecker.py::testfunc_Dict_arg tests/test_typechecker.py::testfunc_Mapping_arg tests/test_typechecker.py::testfunc_Dict_ret tests/test_typechecker.py::testfunc_Dict_ret_err tests/test_typechecker.py::testfunc_Seq_arg tests/test_typechecker.py::testfunc_Seq_ret_List tests/test_typechecker.py::testfunc_Seq_ret_Tuple tests/test_typechecker.py::testfunc_Seq_ret_err tests/test_typechecker.py::testfunc_Iter_arg tests/test_typechecker.py::testfunc_Iter_str_arg tests/test_typechecker.py::testfunc_Iter_ret_err tests/test_typechecker.py::testfunc_Callable_arg tests/test_typechecker.py::testfunc_Callable_call_err tests/test_typechecker.py::testfunc_Callable_ret tests/test_typechecker.py::testfunc_Callable_ret_err tests/test_typechecker.py::testfunc_Generator_arg tests/test_typechecker.py::testfunc_Generator_ret tests/test_typechecker.py::testfunc_Generic_arg tests/test_typechecker.py::testfunc_Generic_ret tests/test_typechecker.py::testfunc_Generic_ret_err tests/test_typechecker.py::testfunc_numeric_tower_float tests/test_typechecker.py::testfunc_numeric_tower_complex tests/test_typechecker.py::testfunc_numeric_tower_tuple tests/test_typechecker.py::testfunc_numeric_tower_return tests/test_typechecker.py::testfunc_numeric_tower_return_err tests/test_typechecker.py::testfunc_custom_annotations_typechecked tests/test_typechecker.py::testfunc_custom_annotations_typechecked_err tests/test_typechecker.py::testfunc_varargs2 tests/test_typechecker.py::testfunc_varargs3 tests/test_typechecker.py::testfunc_varargs5 tests/test_typechecker.py::testfunc_varargs_err tests/test_typechecker.py::testfunc_varargs_ca3
tests/test_typechecker.py::testfunc_Iter_ret tests/test_typechecker.py::testfunc_Generator tests/test_typechecker.py::testfunc_varargs1 tests/test_typechecker.py::testfunc_varargs4 tests/test_typechecker.py::testClass2_defTimeCheck tests/test_typechecker.py::testClass2_defTimeCheck2 tests/test_typechecker.py::testClass2_defTimeCheck3 tests/test_typechecker.py::testClass2_defTimeCheck4 tests/test_typechecker.py::testClass3_defTimeCheck tests/test_typechecker.py::testClass2_defTimeCheck_init_ov tests/test_typechecker.py::testfunc_check_argument_types_empty tests/test_typechecker.py::testfunc_varargs_ca1 tests/test_typechecker.py::testfunc_varargs_ca4 tests/test_typechecker.py::TestTypecheck::test_abstract_override tests/test_typechecker.py::TestTypecheck::test_annotations_from_typestring tests/test_typechecker.py::TestTypecheck::test_callable tests/test_typechecker.py::TestTypecheck::test_classmethod tests/test_typechecker.py::TestTypecheck::test_custom_annotations tests/test_typechecker.py::TestTypecheck::test_custom_generic tests/test_typechecker.py::TestTypecheck::test_defaults_inferred_types tests/test_typechecker.py::TestTypecheck::test_dict tests/test_typechecker.py::TestTypecheck::test_empty tests/test_typechecker.py::TestTypecheck::test_function tests/test_typechecker.py::TestTypecheck::test_generator tests/test_typechecker.py::TestTypecheck::test_get_generic_parameters tests/test_typechecker.py::TestTypecheck::test_get_types tests/test_typechecker.py::TestTypecheck::test_iterable tests/test_typechecker.py::TestTypecheck::test_method tests/test_typechecker.py::TestTypecheck::test_method_forward tests/test_typechecker.py::TestTypecheck::test_numeric_tower tests/test_typechecker.py::TestTypecheck::test_parent_typecheck_no_override tests/test_typechecker.py::TestTypecheck::test_parent_typecheck_other_signature tests/test_typechecker.py::TestTypecheck::test_property tests/test_typechecker.py::TestTypecheck::test_sequence tests/test_typechecker.py::TestTypecheck::test_staticmethod tests/test_typechecker.py::TestTypecheck::test_subtype_class_extends_generic tests/test_typechecker.py::TestTypecheck::test_typecheck_parent_type tests/test_typechecker.py::TestTypecheck::test_typestring_varargs_syntax tests/test_typechecker.py::TestTypecheck::test_typevar_class tests/test_typechecker.py::TestTypecheck::test_typevar_func tests/test_typechecker.py::TestTypecheck::test_unparameterized tests/test_typechecker.py::TestTypecheck::test_varargs tests/test_typechecker.py::TestTypecheck::test_varargs_check_argument_types tests/test_typechecker.py::TestTypecheck::test_various tests/test_typechecker.py::TestTypecheck_class::test_classmethod tests/test_typechecker.py::TestTypecheck_class::test_method tests/test_typechecker.py::TestTypecheck_class::test_staticmethod tests/test_typechecker.py::TestTypecheck_module::test_function_py2 tests/test_typechecker.py::TestTypecheck_module::test_function_py3 tests/test_typechecker.py::Test_check_argument_types::test_function tests/test_typechecker.py::Test_check_argument_types::test_inner_class tests/test_typechecker.py::Test_check_argument_types::test_inner_method tests/test_typechecker.py::Test_check_argument_types::test_methods tests/test_typechecker.py::TestOverride::test_auto_override tests/test_typechecker.py::TestOverride::test_override tests/test_typechecker.py::TestOverride::test_override_at_definition_time tests/test_typechecker.py::TestOverride::test_override_at_definition_time_with_forward_decl tests/test_typechecker.py::TestOverride::test_override_diamond tests/test_typechecker.py::TestOverride::test_override_typecheck tests/test_typechecker.py::TestOverride::test_override_typecheck_class tests/test_typechecker.py::TestOverride::test_override_vararg tests/test_typechecker.py::TestStubfile::test_annotations_from_stubfile_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_annotations_from_stubfile_plain_3_5_stub tests/test_typechecker.py::TestStubfile::test_callable_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_custom_generic_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_defaults_inferred_types_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_defaults_inferred_types_plain_3_5_stub tests/test_typechecker.py::TestStubfile::test_dict_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_generator_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_iterable_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_override_diamond_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_override_diamond_plain_3_5_stub tests/test_typechecker.py::TestStubfile::test_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_plain_3_5_stub tests/test_typechecker.py::TestStubfile::test_property_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_property_plain_3_5_stub tests/test_typechecker.py::TestStubfile::test_sequence_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_typecheck_parent_type_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_typecheck_parent_type_plain_3_5_stub tests/test_typechecker.py::TestStubfile::test_varargs_check_argument_types_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_varargs_check_argument_types_plain_3_5_stub tests/test_typechecker.py::TestStubfile::test_varargs_plain_2_7_stub tests/test_typechecker.py::TestStubfile::test_varargs_plain_3_5_stub tests/test_typechecker.py::TestTypecheck_Python3_5::test_abstract_override_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_callable_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_classmethod_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_custom_generic_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_defaults_inferred_types tests/test_typechecker.py::TestTypecheck_Python3_5::test_dict_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_function_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_generator_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_get_types_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_iterable_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_method_forward_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_method_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_parent_typecheck_no_override_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_parent_typecheck_other_signature_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_property tests/test_typechecker.py::TestTypecheck_Python3_5::test_sequence_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_staticmethod_py3 tests/test_typechecker.py::TestTypecheck_Python3_5::test_typecheck_parent_type tests/test_typechecker.py::TestTypecheck_Python3_5::test_typevar_func tests/test_typechecker.py::TestTypecheck_Python3_5::test_varargs tests/test_typechecker.py::TestTypecheck_Python3_5::test_varargs_check_argument_types tests/test_typechecker.py::TestTypecheck_Python3_5::test_various_py3 tests/test_typechecker.py::TestOverride_Python3_5::test_auto_override tests/test_typechecker.py::TestOverride_Python3_5::test_override_at_definition_time tests/test_typechecker.py::TestOverride_Python3_5::test_override_at_definition_time_with_forward_decl tests/test_typechecker.py::TestOverride_Python3_5::test_override_diamond tests/test_typechecker.py::TestOverride_Python3_5::test_override_py3 tests/test_typechecker.py::TestOverride_Python3_5::test_override_typecheck tests/test_typechecker.py::TestOverride_Python3_5::test_override_vararg tests/test_typechecker.py::Test_check_argument_types_Python3_5::test_function tests/test_typechecker.py::Test_check_argument_types_Python3_5::test_inner_class tests/test_typechecker.py::Test_check_argument_types_Python3_5::test_inner_method tests/test_typechecker.py::Test_check_argument_types_Python3_5::test_methods tests/test_typechecker.py::Test_utils::test_resolve_fw_decl
pytypes/type_util.py
pytypes/type_util.py