diff --git a/Lib/multiprocessing/reduction.py b/Lib/multiprocessing/reduction.py
index 473fd59..5593f06 100644
--- a/Lib/multiprocessing/reduction.py
+++ b/Lib/multiprocessing/reduction.py
@@ -154,7 +154,7 @@
         '''Receive an array of fds over an AF_UNIX socket.'''
         a = array.array('i')
         bytes_size = a.itemsize * size
-        msg, ancdata, flags, addr = sock.recvmsg(1, socket.CMSG_LEN(bytes_size))
+        msg, ancdata, flags, addr = sock.recvmsg(1, socket.CMSG_SPACE(bytes_size))
         if not msg and not ancdata:
             raise EOFError
         try: