Index: Lib/test/test_mmap.py
===================================================================
--- Lib/test/test_mmap.py	(revision 70800)
+++ Lib/test/test_mmap.py	(working copy)
@@ -354,6 +354,7 @@
         # should not crash
         m = mmap.mmap(-1, 1)
         self.assertRaises(ValueError, m.move, 1, 1, -1)
+        self.assertRaises(ValueError, m.move, -1, -1, 1)
         m.close()
 
     def test_anonymous(self):
Index: Modules/mmapmodule.c
===================================================================
--- Modules/mmapmodule.c	(revision 70800)
+++ Modules/mmapmodule.c	(working copy)
@@ -613,7 +613,7 @@
 	} else {
 		/* bounds check the values */
 		unsigned long pos = src > dest ? src : dest;
-		if (self->size >= pos && count > self->size - pos) {
+		if (self->size < pos || count > self->size - pos) {
 			PyErr_SetString(PyExc_ValueError,
 					"source or destination out of range");
 			return NULL;