diff --git a/Lib/ctypes/test/test_callbacks.py b/Lib/ctypes/test/test_callbacks.py
index 4b468b9e11f..8801ccd1069 100644
--- a/Lib/ctypes/test/test_callbacks.py
+++ b/Lib/ctypes/test/test_callbacks.py
@@ -200,6 +200,42 @@ class SampleCallbacksTestCase(unittest.TestCase):
 
             windll.user32.EnumWindows(EnumWindowsCallbackFunc, 0)
 
+    def test_callback_register_int(self):
+        # Issue #8275: buggy handling of callback args under Win64
+        # NOTE: should be run on release builds as well
+        dll = CDLL(_ctypes_test.__file__)
+        CALLBACK = CFUNCTYPE(c_int, c_int, c_int, c_int, c_int, c_int)
+        # All this function does is call the callback with its args squared
+        func = dll._testfunc_cbk_reg_int
+        func.argtypes = (c_int, c_int, c_int, c_int, c_int, CALLBACK)
+        func.restype = c_int
+
+        def callback(a, b, c, d, e):
+            return a + b + c + d + e
+
+        result = func(2, 3, 4, 5, 6, CALLBACK(callback))
+        self.assertEqual(result, callback(2*2, 3*3, 4*4, 5*5, 6*6))
+
+    def test_callback_register_double(self):
+        # Issue #8275: buggy handling of callback args under Win64
+        # NOTE: should be run on release builds as well
+        dll = CDLL(_ctypes_test.__file__)
+        CALLBACK = CFUNCTYPE(c_double, c_double, c_double, c_double,
+                             c_double, c_double)
+        # All this function does is call the callback with its args squared
+        func = dll._testfunc_cbk_reg_double
+        func.argtypes = (c_double, c_double, c_double,
+                         c_double, c_double, CALLBACK)
+        func.restype = c_double
+
+        def callback(a, b, c, d, e):
+            return a + b + c + d + e
+
+        result = func(1.1, 2.2, 3.3, 4.4, 5.5, CALLBACK(callback))
+        self.assertEqual(result,
+                         callback(1.1*1.1, 2.2*2.2, 3.3*3.3, 4.4*4.4, 5.5*5.5))
+
+
 ################################################################
 
 if __name__ == '__main__':
diff --git a/Misc/ACKS b/Misc/ACKS
index 02e5d9168dd..8e120076540 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -574,6 +574,7 @@ Luke Mewburn
 Mike Meyer
 Steven Miale
 Trent Mick
+Stan Mihai
 Aristotelis Mikropoulos
 Damien Miller
 Chad Miller
diff --git a/Misc/NEWS b/Misc/NEWS
index 6b6fae900dd..44a71d8145a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -13,6 +13,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #8275: Fix passing of callback arguments with ctypes under Win64.
+  Patch by Stan Mihai.
+
 
 What's New in Python 3.2 Release Candidate 2?
 =============================================
diff --git a/Modules/_ctypes/_ctypes_test.c b/Modules/_ctypes/_ctypes_test.c
index 36bd0a1465b..3a6845f39bb 100644
--- a/Modules/_ctypes/_ctypes_test.c
+++ b/Modules/_ctypes/_ctypes_test.c
@@ -12,6 +12,20 @@
 
 /* some functions handy for testing */
 
+EXPORT(int)
+_testfunc_cbk_reg_int(int a, int b, int c, int d, int e,
+                      int (*func)(int, int, int, int, int))
+{
+    return func(a*a, b*b, c*c, d*d, e*e);
+}
+
+EXPORT(double)
+_testfunc_cbk_reg_double(double a, double b, double c, double d, double e,
+                         double (*func)(double, double, double, double, double))
+{
+    return func(a*a, b*b, c*c, d*d, e*e);
+}
+
 EXPORT(void)testfunc_array(int values[4])
 {
     printf("testfunc_array %d %d %d %d\n",
diff --git a/Modules/_ctypes/libffi_msvc/ffi.c b/Modules/_ctypes/libffi_msvc/ffi.c
index 5df4ba4be50..6e595e9fe74 100644
--- a/Modules/_ctypes/libffi_msvc/ffi.c
+++ b/Modules/_ctypes/libffi_msvc/ffi.c
@@ -380,7 +380,7 @@ ffi_prep_closure_loc (ffi_closure* closure,
   short bytes;
   char *tramp;
 #ifdef _WIN64
-  int mask;
+  int mask = 0;
 #endif
   FFI_ASSERT (cif->abi == FFI_SYSV);