diff --git a/filter/ngx_http_zstd_filter_module.c b/filter/ngx_http_zstd_filter_module.c
index 6549382..d5784ba 100644
--- a/filter/ngx_http_zstd_filter_module.c
+++ b/filter/ngx_http_zstd_filter_module.c
@@ -641,7 +641,7 @@ ngx_http_zstd_accept_encoding(ngx_str_t *ae)
 {
     u_char  *p;
 
-    p = ngx_strcasestrn(ae->data, "zstd", sizeof("zstd") - 1);
+    p = ngx_strcasestrn(ae->data, "zstd", sizeof("zstd") - 2);
     if (p == NULL) {
         return NGX_DECLINED;
     }
@@ -1014,4 +1014,4 @@ ngx_conf_zstd_set_num_slot_with_negatives(ngx_conf_t *cf, ngx_command_t *cmd, vo
     }
 
     return NGX_CONF_OK;
-}
\ No newline at end of file
+}