diff --git a/libratbox/src/commio.c b/libratbox/src/commio.c
index 7318cabe..cd1f5a07 100644
--- a/libratbox/src/commio.c
+++ b/libratbox/src/commio.c
@@ -2156,7 +2156,7 @@ rb_recv_fd_buf(rb_fde_t *F, void *data, size_t datasize, rb_fde_t **xF, int nfds
 	if(msg.msg_controllen > 0 && msg.msg_control != NULL
 	   && (cmsg = CMSG_FIRSTHDR(&msg)) != NULL)
 	{
-		rfds = (msg.msg_controllen - sizeof(struct cmsghdr)) / sizeof(int);
+		rfds = ((unsigned char *)cmsg + cmsg->cmsg_len - CMSG_DATA(cmsg)) / sizeof(int);
 
 		for(x = 0; x < nfds && x < rfds; x++)
 		{