diff --git a/Lib/ctypes/test/test_prototypes.py b/Lib/ctypes/test/test_prototypes.py
index 2c3d75b5227..47f5da1091b 100644
--- a/Lib/ctypes/test/test_prototypes.py
+++ b/Lib/ctypes/test/test_prototypes.py
@@ -24,6 +24,19 @@ import unittest
 import _ctypes_test
 testdll = cdll.load(_ctypes_test.__file__)
 
+# Return machine address `a` as a (possibly long) non-negative integer.
+# Starting with Python 2.5, id(anything) is always non-negative, and
+# the ctypes addressof() inherits that via PyLong_FromVoidPtr().
+def positive_address(a):
+    if a >= 0:
+        return a
+    # View the bits in `a` as unsigned instead.
+    import struct
+    num_bits = struct.calcsize("P") * 8 # num bits in native machine address
+    a += 1L << num_bits
+    assert a >= 0
+    return a
+
 def c_wbuffer(init):
     n = len(init) + 1
     return (c_wchar * n)(*init)
@@ -43,7 +56,8 @@ class CharPointersTestCase(unittest.TestCase):
         ci = c_int(0)
 
         func.argtypes = POINTER(c_int),
-        self.failUnlessEqual(addressof(ci), func(byref(ci)))
+        self.failUnlessEqual(positive_address(addressof(ci)),
+                             positive_address(func(byref(ci))))
 
         func.argtypes = c_char_p,
         self.assertRaises(ArgumentError, func, byref(ci))