diff -r 90a06fbb1f85 Lib/test/test_io.py
--- a/Lib/test/test_io.py	Sat Oct 01 19:22:30 2011 +0200
+++ b/Lib/test/test_io.py	Sat Oct 01 18:25:21 2011 -0400
@@ -928,6 +928,14 @@
         finally:
             support.unlink(support.TESTFN)
 
+    def test_unseekable(self):
+        bufio = self.tp(self.MockUnseekableIO(b"A" * 10))
+        self.assertRaises(self.UnsupportedOperation, bufio.tell)
+        self.assertRaises(self.UnsupportedOperation, bufio.seek, 0)
+        bufio.read(1)
+        self.assertRaises(self.UnsupportedOperation, bufio.seek, 0)
+        self.assertRaises(self.UnsupportedOperation, bufio.tell)
+
     def test_misbehaved_io(self):
         rawio = self.MisbehavedRawIO((b"abc", b"d", b"efg"))
         bufio = self.tp(rawio)
diff -r 90a06fbb1f85 Modules/_io/bufferedio.c
--- a/Modules/_io/bufferedio.c	Sat Oct 01 19:22:30 2011 +0200
+++ b/Modules/_io/bufferedio.c	Sat Oct 01 18:25:21 2011 -0400
@@ -1155,6 +1155,9 @@
 
     CHECK_CLOSED(self, "seek of closed file")
 
+    if (_PyIOBase_check_seekable(self->raw, Py_True) == NULL)
+        return NULL;
+
     target = PyNumber_AsOff_t(targetobj, PyExc_ValueError);
     if (target == -1 && PyErr_Occurred())
         return NULL;