diff --git a/CHANGES.rst b/CHANGES.rst
index 01e6173..b1b697e 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -5,7 +5,10 @@
 4.9.1 (unreleased)
 ==================
 
-- Nothing changed yet.
+- Fix setting unknown class attributes on subclasses of BTrees when
+  using the C extension. This prevented subclasses from being
+  decorated with ``@component.adapter()``. See `issue 168
+  <https://github.com/zopefoundation/BTrees/issues/168>`_.
 
 
 4.9.0 (2021-05-26)
diff --git a/src/BTrees/BTreeTemplate.c b/src/BTrees/BTreeTemplate.c
index 192730c..ec7c021 100644
--- a/src/BTrees/BTreeTemplate.c
+++ b/src/BTrees/BTreeTemplate.c
@@ -2532,21 +2532,7 @@ BTreeType_setattro(PyTypeObject* type, PyObject* name, PyObject* value)
         }
         return 0;
     }
-    PyErr_Format(
-        PyExc_TypeError,
-#ifndef PY3K
-        /* Python 2 doesn't have %R. Both branches otherwise match */
-        /* what the standard type object would produce, distinguished  */
-        /* by the BTrees prefix. */
-        "BTrees: can't set attribute of built-in/extension type '%s'",
-        type->tp_name
-#else
-        "BTrees: cannot set attribute %R of immutable type '%s'",
-        name,
-        type->tp_name
-#endif
-        );
-    return -1;
+    return PyType_Type.tp_setattro((PyObject*)type, name, value);
 }
 
 static PyTypeObject BTreeTypeType = {
diff --git a/src/BTrees/tests/common.py b/src/BTrees/tests/common.py
index 2a34c3c..7128e43 100644
--- a/src/BTrees/tests/common.py
+++ b/src/BTrees/tests/common.py
@@ -138,6 +138,22 @@ def setUp(self):
         super(Base, self).setUp()
         _skip_if_pure_py_and_py_test(self)
 
+    def testSubclassesCanHaveAttributes(self):
+        # https://github.com/zopefoundation/BTrees/issues/168
+        class Subclass(self._getTargetClass()):
+            pass
+        Subclass.foo = 1
+        self.assertIn('foo', Subclass.__dict__)
+        self.assertNotIn('foo', self._getTargetClass().__dict__)
+
+    @skipOnPurePython
+    def testCannotSetArbitraryAttributeOnBase(self):
+        if 'Py' in self._getTargetClass().__name__:
+            # pure-python classes can have arbitrary attributes
+            self.skipTest("No on Pure Python.")
+        with self.assertRaises(TypeError):
+            self._getTargetClass().foo = 1
+
     def testProvidesInterface(self):
         from zope.interface import providedBy
         from zope.interface.common.sequence import IMinimalSequence