diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index 4e27917..9d854fa 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -1407,7 +1407,7 @@
 {
     ngx_uint_t      n;
     uint32_t       *escape;
-    static u_char   hex[] = "0123456789abcdef";
+    static u_char   hex[] = "0123456789ABCDEF";
 
                     /* " ", "#", "%", "?", %00-%1F, %7F-%FF */