diff --git a/src/event/ngx_event_pipe.h b/src/event/ngx_event_pipe.h
index 961f066..9781820 100644
--- a/src/event/ngx_event_pipe.h
+++ b/src/event/ngx_event_pipe.h
@@ -62,7 +62,7 @@
     ngx_bufs_t         bufs;
     ngx_buf_tag_t      tag;
 
-    size_t             busy_size;
+    ssize_t            busy_size;
 
     off_t              read_length;