diff --git a/src/http/v3/ngx_http_v3_parse.c b/src/http/v3/ngx_http_v3_parse.c
index 96f87b0b6..6ce5f10a7 100644
--- a/src/http/v3/ngx_http_v3_parse.c
+++ b/src/http/v3/ngx_http_v3_parse.c
@@ -212,6 +212,10 @@ ngx_http_v3_parse_headers(ngx_connection_t *c, ngx_http_v3_parse_headers_t *st,
             break;
         }
 
+        if (st->length == 0) {
+            return NGX_HTTP_V3_ERR_FRAME_ERROR;
+        }
+
         st->state = sw_prefix;
         break;
 
@@ -225,7 +229,7 @@ ngx_http_v3_parse_headers(ngx_connection_t *c, ngx_http_v3_parse_headers_t *st,
 
     case sw_prefix:
 
-        if (st->length-- == 0) {
+        if (--st->length == 0) {
             return NGX_HTTP_V3_ERR_FRAME_ERROR;
         }
 
@@ -234,10 +238,6 @@ ngx_http_v3_parse_headers(ngx_connection_t *c, ngx_http_v3_parse_headers_t *st,
             return rc;
         }
 
-        if (st->length == 0) {
-            return NGX_HTTP_V3_ERR_FRAME_ERROR;
-        }
-
         st->state = sw_verify;
         break;