diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index d547ca74e..d574a6a31 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -441,6 +441,7 @@ ngx_http_upstream_create(ngx_http_request_t *r)
 #endif
 
     u->headers_in.content_length_n = -1;
+    u->headers_in.last_modified_time = -1;
 
     return NGX_OK;
 }
@@ -885,6 +886,7 @@ ngx_http_upstream_cache_send(ngx_http_request_t *r, ngx_http_upstream_t *u)
 
     ngx_memzero(&u->headers_in, sizeof(ngx_http_upstream_headers_in_t));
     u->headers_in.content_length_n = -1;
+    u->headers_in.last_modified_time = -1;
 
     if (ngx_list_init(&u->headers_in.headers, r->pool, 8,
                       sizeof(ngx_table_elt_t))
@@ -1582,6 +1584,7 @@ ngx_http_upstream_reinit(ngx_http_request_t *r, ngx_http_upstream_t *u)
 
     ngx_memzero(&u->headers_in, sizeof(ngx_http_upstream_headers_in_t));
     u->headers_in.content_length_n = -1;
+    u->headers_in.last_modified_time = -1;
 
     if (ngx_list_init(&u->headers_in.headers, r->pool, 8,
                       sizeof(ngx_table_elt_t))