diff --git a/Lib/ctypes/test/test_bitfields.py b/Lib/ctypes/test/test_bitfields.py
index e9c46c2..211d1db 100644
--- a/Lib/ctypes/test/test_bitfields.py
+++ b/Lib/ctypes/test/test_bitfields.py
@@ -240,5 +240,20 @@
             _anonymous_ = ["_"]
             _fields_ = [("_", X)]
 
+    def test_uint32(self):
+        class X(Structure):
+            _fields_ = [("a", c_uint32, 32)]
+        x = X()
+        x.a = 10
+        self.failUnlessEqual(x.a, 10)
+
+    def test_uint64(self):
+        class X(Structure):
+            _fields_ = [("a", c_uint64, 64)]
+        x = X()
+        x.a = 10
+        self.failUnlessEqual(x.a, 10)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS b/Misc/NEWS
index 77e3ac6..d175786 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -349,6 +349,8 @@
 Library
 -------
 
+- Issue #6493: Fix a ctypes problem setting bitfields more than 31 bits wide.
+
 - unittest has been split up into a package.  All old names should still work.
 
 - Issue #6431: Make Fraction type return NotImplemented when it doesn't
diff --git a/Modules/_ctypes/cfield.c b/Modules/_ctypes/cfield.c
index bf247bc..7663481 100644
--- a/Modules/_ctypes/cfield.c
+++ b/Modules/_ctypes/cfield.c
@@ -426,9 +426,9 @@
 #define LOW_BIT(x)  ((x) & 0xFFFF)
 #define NUM_BITS(x) ((x) >> 16)
 
-/* This seems nore a compiler issue than a Windows/non-Windows one */
+/* This seems more a compiler issue than a Windows/non-Windows one */
 #ifdef MS_WIN32
-#  define BIT_MASK(size) ((1 << NUM_BITS(size))-1)
+#  define BIT_MASK(size) ((1i64 << NUM_BITS(size))-1)
 #else
 #  define BIT_MASK(size) ((1LL << NUM_BITS(size))-1)
 #endif