diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index ffe7fb4..47e5ff7 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -1933,6 +1933,12 @@
 ngx_int_t
 ngx_http_send_header(ngx_http_request_t *r)
 {
+    if (r->header_sent) {
+        ngx_log_error(NGX_LOG_ALERT, r->connection->log, 0,
+                      "header already sent");
+        return NGX_ERROR;
+    }
+
     if (r->err_status) {
         r->headers_out.status = r->err_status;
         r->headers_out.status_line.len = 0;