diff --git a/lib/libthr/thread/thr_private.h b/lib/libthr/thread/thr_private.h
index 4f61b196563..4855b25134a 100644
--- a/lib/libthr/thread/thr_private.h
+++ b/lib/libthr/thread/thr_private.h
@@ -705,7 +705,7 @@ ssize_t __sys_write(int, const void *, size_t);
 void	__sys_exit(int);
 #endif
 
-int	_umtx_op_err(void *, int op, u_long, void *, void *);
+int	_umtx_op_err(void *, int op, u_long, void *, void *) __hidden;
 
 static inline int
 _thr_isthreaded(void)