diff --git a/src/http/ngx_http_upstream.h b/src/http/ngx_http_upstream.h
index f3e9f7979..5e14666ad 100644
--- a/src/http/ngx_http_upstream.h
+++ b/src/http/ngx_http_upstream.h
@@ -21,6 +21,7 @@
 #define NGX_HTTP_UPSTREAM_FT_TIMEOUT         0x00000004
 #define NGX_HTTP_UPSTREAM_FT_INVALID_HEADER  0x00000008
 #define NGX_HTTP_UPSTREAM_FT_HTTP_500        0x00000010
+#define NGX_HTTP_UPSTREAM_FT_HTTP_501        0x00008000
 #define NGX_HTTP_UPSTREAM_FT_HTTP_502        0x00000020
 #define NGX_HTTP_UPSTREAM_FT_HTTP_503        0x00000040
 #define NGX_HTTP_UPSTREAM_FT_HTTP_504        0x00000080
@@ -35,6 +36,7 @@
 #define NGX_HTTP_UPSTREAM_FT_OFF             0x80000000
 
 #define NGX_HTTP_UPSTREAM_FT_STATUS          (NGX_HTTP_UPSTREAM_FT_HTTP_500  \
+                                             |NGX_HTTP_UPSTREAM_FT_HTTP_501  \
                                              |NGX_HTTP_UPSTREAM_FT_HTTP_502  \
                                              |NGX_HTTP_UPSTREAM_FT_HTTP_503  \
                                              |NGX_HTTP_UPSTREAM_FT_HTTP_504  \