don't ignore exceptions from PyObject_IsTrue
diff --git a/Lib/test/test_struct.py b/Lib/test/test_struct.py
index 8448286..3168a7b 100644
--- a/Lib/test/test_struct.py
+++ b/Lib/test/test_struct.py
@@ -475,6 +475,9 @@
             self.assertEqual(value, 0x12345678)
 
     def test_bool(self):
+        class ExplodingBool(object):
+            def __bool__(self):
+                raise IOError
         for prefix in tuple("<>!=")+('',):
             false = (), [], [], '', 0
             true = [1], 'test', 5, -1, 0xffffffff+1, 0xffffffff/2
@@ -503,6 +506,9 @@
                 self.assertFalse(prefix, msg='encoded bool is not one byte: %r'
                                              %packed)
 
+            self.assertRaises(IOError, struct.pack, prefix + '?',
+                              ExplodingBool())
+
         for c in [b'\x01', b'\x7f', b'\xff', b'\x0f', b'\xf0']:
             self.assertTrue(struct.unpack('>?', c)[0])