diff --git a/src/tools/tor-fw-helper/tor-fw-helper-natpmp.c b/src/tools/tor-fw-helper/tor-fw-helper-natpmp.c
index a8cdbbf24a6842054dbac02bd296fb3c71b9bfaf..aaa685e1ee49698c4759751621c84102dfe1b103 100644
--- a/src/tools/tor-fw-helper/tor-fw-helper-natpmp.c
+++ b/src/tools/tor-fw-helper/tor-fw-helper-natpmp.c
@@ -84,7 +84,7 @@ tor_natpmp_cleanup(tor_fw_options_t *tor_fw_options, void *backend_state)
 
 /** Use select() to wait until we can read on fd. */
 static int
-wait_until_fd_readable(int fd, struct timeval *timeout)
+wait_until_fd_readable(unsigned int fd, struct timeval *timeout)
 {
   int r;
   fd_set fds;