diff --git a/src/os/unix/ngx_thread.h b/src/os/unix/ngx_thread.h
index 0b065d8..a85750d 100644
--- a/src/os/unix/ngx_thread.h
+++ b/src/os/unix/ngx_thread.h
@@ -103,7 +103,7 @@
 #define ngx_log_tid  0
 #define TID_T_FMT    "%d"
 
-#define ngx_mutex_lock(m)
+#define ngx_mutex_lock(m)     NGX_OK
 #define ngx_mutex_unlock(m)
 
 #endif