diff --git a/include/proto/fd.h b/include/proto/fd.h
index 8fa7670f9..9783f9072 100644
--- a/include/proto/fd.h
+++ b/include/proto/fd.h
@@ -67,7 +67,7 @@ void fd_remove(int fd);
 int fd_takeover(int fd, void *expected_owner);
 
 #ifndef HA_HAVE_CAS_DW
-__decl_hathreads(HA_RWLOCK_T fd_mig_lock);
+__decl_hathreads(extern HA_RWLOCK_T fd_mig_lock);
 #endif
 
 ssize_t fd_write_frag_line(int fd, size_t maxlen, const struct ist pfx[], size_t npfx, const struct ist msg[], size_t nmsg, int nl);