diff --git a/configure.ac b/configure.ac
index 5e4b4029..0501a02d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -116,7 +116,7 @@ AC_PROG_CXX
 # Compiler flags.
 AC_ARG_ENABLE(debug, [AC_HELP_STRING([--enable-debug], [setup flags (gcc) for debugging (default=no)])],
 if test "x$enable_debug" = xyes; then
-  CFLAGS="$CFLAGS -O2 -g"
+  CFLAGS="$CFLAGS -O0 -g"
 fi
 CPPFLAGS="$CPPFLAGS -DDEBUG"
 LDFLAGS="$LDFLAGS",)
diff --git a/src/data-types/mailstream_ssl.c b/src/data-types/mailstream_ssl.c
index bd680ae5..d6494312 100644
--- a/src/data-types/mailstream_ssl.c
+++ b/src/data-types/mailstream_ssl.c
@@ -338,6 +338,13 @@ static int wait_SSL_connect(int s, int want_read, time_t timeout_seconds)
     timeout.tv_usec = 0;
   }
 #if defined(WIN32) || !USE_POLL
+  if (s >= FD_SETSIZE) {
+#if defined(DEBUG)
+    /* too many opened connections? socket leaks? */
+    fprintf( stderr, "socket descriptor %d out of range\n", s);
+#endif
+    return -1;
+  }
   FD_ZERO(&fds);
   FD_SET(s, &fds);
   /* TODO: how to cancel this ? */