diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index cfd4583..39fda2b 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -611,7 +611,24 @@
         self.assertEqual(rawio.read(2), b"")
 
 class CIOTest(IOTest):
-    pass
+
+    def test_IOBase_finalize(self):
+        # Issue #12149: segmentation fault on _PyIOBase_finalize when both a
+        # class which inherits IOBase and an object of this class are caught
+        # in a reference cycle and close() is already in the method cache.
+        class MyIO(self.IOBase):
+            def close(self):
+                pass
+
+        # create an instance to populate the method cache
+        MyIO()
+        obj = MyIO()
+        obj.obj = obj
+        wr = weakref.ref(obj)
+        del MyIO
+        del obj
+        support.gc_collect()
+        self.assertTrue(wr() is None, wr)
 
 class PyIOTest(IOTest):
     pass
diff --git a/Misc/ACKS b/Misc/ACKS
index 45b4042..625317d 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -736,6 +736,7 @@
 Nicholas Riley
 Jean-Claude Rimbault
 Juan M. Bello Rivas
+Davide Rizzo
 Anthony Roach
 Mark Roberts
 Jim Robinson
diff --git a/Misc/NEWS b/Misc/NEWS
index 9a46b72..3df8e95 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,12 @@
 Core and Builtins
 -----------------
 
+- Issue #12149: Update the method cache after a type's dictionnary gets
+  cleared by the garbage collector.  This fixes a segfault when an instance
+  and its type get caught in a reference cycle, and the instance's
+  deallocator calls one of the methods on the type (e.g. when subclassing
+  IOBase).  Diagnosis and patch by Davide Rizzo.
+
 - Issue #9611, #9015: FileIO.read() clamps the length to INT_MAX on Windows.
 
 - When a generator yields, do not retain the caller's exception state on the
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 3c724fd..97ccf01 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -967,6 +967,8 @@
     assert(basedealloc);
     basedealloc(self);
 
+    PyType_Modified(type);
+
     /* Can't reference self beyond this point */
     Py_DECREF(type);