diff -r 768234f5c246 Lib/multiprocessing/pool.py
--- a/Lib/multiprocessing/pool.py	Sat Jun 25 15:03:52 2011 +0200
+++ b/Lib/multiprocessing/pool.py	Sat Jun 25 18:46:01 2011 +0200
@@ -347,17 +347,16 @@ class Pool(object):
                 if set_length:
                     debug('doing set_length()')
                     set_length(i+1)
                 continue
             break
         else:
             debug('task handler got sentinel')
 
-
         try:
             # tell result handler to finish when cache is empty
             debug('task handler sending sentinel to result handler')
             outqueue.put(None)
 
             # tell workers there is no more work
             debug('task handler sending sentinel to workers')
             for p in pool:
@@ -575,16 +574,17 @@ class MapResult(ApplyResult):
         ApplyResult.__init__(self, cache, callback,
                              error_callback=error_callback)
         self._success = True
         self._value = [None] * length
         self._chunksize = chunksize
         if chunksize <= 0:
             self._number_left = 0
             self._ready = True
+            del cache[self._job]
         else:
             self._number_left = length//chunksize + bool(length % chunksize)
 
     def _set(self, i, success_result):
         success, result = success_result
         if success:
             self._value[i*self._chunksize:(i+1)*self._chunksize] = result
             self._number_left -= 1
diff -r 768234f5c246 Lib/test/test_multiprocessing.py
--- a/Lib/test/test_multiprocessing.py	Sat Jun 25 15:03:52 2011 +0200
+++ b/Lib/test/test_multiprocessing.py	Sat Jun 25 18:46:01 2011 +0200
@@ -1093,16 +1093,22 @@ class _TestPool(BaseTestCase):
         self.assertEqual(papply(sqr, (), {'x':3}), sqr(x=3))
 
     def test_map(self):
         pmap = self.pool.map
         self.assertEqual(pmap(sqr, list(range(10))), list(map(sqr, list(range(10)))))
         self.assertEqual(pmap(sqr, list(range(100)), chunksize=20),
                          list(map(sqr, list(range(100)))))
 
+        # Pass an empty sequence to pool.map issue #12157.
+        p = multiprocessing.Pool(1)
+        p.map(sqr, [])
+        p.close()
+        p.join()
+
     def test_map_chunksize(self):
         try:
             self.pool.map_async(sqr, [], chunksize=1).get(timeout=TIMEOUT1)
         except multiprocessing.TimeoutError:
             self.fail("pool.map_async with chunksize stalled on null list")
 
     def test_async(self):
         res = self.pool.apply_async(sqr, (7, TIMEOUT1,))