diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index c30e87a..008258d 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -1,6 +1,11 @@
 Changelog
 =========
 
+0.13.0 (2022-02-26)
+-------------------
+
+- Improved robustness of determining whether a class is pure Python (#65)
+
 0.12.1 (2022-02-12)
 -------------------
 
diff --git a/docs/advanced.rst b/docs/advanced.rst
index 77668f5..4af1e8a 100644
--- a/docs/advanced.rst
+++ b/docs/advanced.rst
@@ -18,7 +18,7 @@ Use the following configuration:
 
    repos:
    - repo: https://github.com/ariebovenberg/slotscheck
-     rev: v0.12.1
+     rev: v0.13.0
      hooks:
      - id: slotscheck
        # If your Python files are not importable from the project root,
diff --git a/pyproject.toml b/pyproject.toml
index 3501694..dcdb6c5 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "slotscheck"
-version = "0.12.1"
+version = "0.13.0"
 description = "Ensure your __slots__ are working properly."
 authors = ["Arie Bovenberg <a.c.bovenberg@gmail.com>"]
 license = "MIT"
diff --git a/src/slotscheck/checks.py b/src/slotscheck/checks.py
index 348f97e..b8dfd4b 100644
--- a/src/slotscheck/checks.py
+++ b/src/slotscheck/checks.py
@@ -1,7 +1,6 @@
 "Slots-related checks and inspection tools"
 import builtins
-import sys
-from functools import lru_cache
+import platform
 from typing import Collection, Iterator, Optional
 
 
@@ -22,10 +21,7 @@ def has_slots(c: type) -> bool:
     return (
         "__slots__" in c.__dict__
         or c in _SLOTTED_BUILTINS
-        or (
-            not issubclass(c, BaseException)
-            and not is_purepython_class(c)  # type: ignore
-        )
+        or (not issubclass(c, BaseException) and not is_pure_python(c))
     )
 
 
@@ -56,26 +52,25 @@ def has_duplicate_slots(c: type) -> bool:
 }
 
 
-_UNSETTABLE_ATTRITUBE_MSG = (
-    "cannot set '_SLOTSCHECK_POKE' attribute of immutable type"
-    if sys.version_info > (3, 10)
-    else "can't set attributes of built-in/extension type"
-)
+# The 'is a pure python class' logic below is adapted
+# from https://stackoverflow.com/a/41012823/
 
+# If the active Python interpreter is the official CPython interpreter,
+# prefer a more reliable CPython-specific solution guaranteed to succeed.
+if platform.python_implementation() == "CPython":
+    # Magic number defined by the Python codebase at "Include/object.h".
+    Py_TPFLAGS_HEAPTYPE = 1 << 9
 
-@lru_cache(maxsize=None)
-def is_purepython_class(t: type) -> bool:
-    "Whether a class is defined in Python, or an extension/C module"
-    # AFIAK there is no _easy_ way to check if a class is pure Python.
-    # One symptom of a non-native class is that it is not possible to
-    # set attributes on it.
-    # Let's use that as an easy proxy for now.
-    try:
-        t._SLOTSCHECK_POKE = 1  # type: ignore
-    except TypeError as e:
-        if e.args[0].startswith(_UNSETTABLE_ATTRITUBE_MSG):
-            return False
-        raise  # some other error we may want to know about
-    else:
-        del t._SLOTSCHECK_POKE  # type: ignore
-        return True
+    def is_pure_python(cls: type) -> bool:
+        "Whether the class is pure-Python or C-based"
+        return bool(cls.__flags__ & Py_TPFLAGS_HEAPTYPE)
+
+
+# Else, fallback to a CPython-agnostic solution typically but *NOT*
+# necessarily succeeding. For all real-world objects of interest, this is
+# effectively successful. Edge cases exist but are suitably rare.
+else:
+
+    def is_pure_python(cls: type) -> bool:
+        "Whether the class is pure-Python or C-based"
+        return "__dict__" in dir(cls) or hasattr(cls, "__slots__")
diff --git a/src/slotscheck/cli.py b/src/slotscheck/cli.py
index 58cacc8..8649692 100644
--- a/src/slotscheck/cli.py
+++ b/src/slotscheck/cli.py
@@ -25,7 +25,7 @@
     has_duplicate_slots,
     has_slotless_base,
     has_slots,
-    is_purepython_class,
+    is_pure_python,
     slots,
     slots_overlap,
 )
@@ -448,7 +448,7 @@ def _print_report(
     classes_by_status = groupby(
         classes,
         key=lambda c: None
-        if not is_purepython_class(c)
+        if not is_pure_python(c)
         else True
         if has_slots(c)
         else False,
diff --git a/tests/src/test_checks.py b/tests/src/test_checks.py
index edeabf3..70db860 100644
--- a/tests/src/test_checks.py
+++ b/tests/src/test_checks.py
@@ -87,9 +87,8 @@ def test_slots(self, klass):
     def test_no_slots(self, klass):
         assert not has_slots(klass)
 
-    def test_opaque_class(self):
-        with pytest.raises(TypeError, match="BOOM!"):
-            assert not has_slots(_UnsettableClass)
+    def test_immutable_class(self):
+        assert not has_slots(_UnsettableClass)
 
 
 class TestSlotsOverlap: