diff --git a/builtin.c b/builtin.c
index de40be7e..bb704bb8 100644
--- a/builtin.c
+++ b/builtin.c
@@ -306,7 +306,7 @@ static jv f_format(jv input, jv fmt) {
     jv line = jv_string("");
     const char* s = jv_string_value(input);
     for (int i=0; i<jv_string_length(jv_copy(input)); i++) {
-      unsigned ch = (unsigned)*s;
+      unsigned ch = (unsigned)(unsigned char)*s;
       if (ch < 128 && unreserved[ch]) {
         line = jv_string_append_buf(line, s, 1);
       } else {
diff --git a/testdata b/testdata
index 36429f59..50e73203 100644
--- a/testdata
+++ b/testdata
@@ -62,6 +62,10 @@ null
 "'<>&'\\''\"'"
 "PD4mJyI="
 
+@uri
+"\u03bc"
+"%ce%bc"
+
 @html "<b>\(.)</b>"
 "<script>hax</script>"
 "<b>&lt;script&gt;hax&lt;/script&gt;</b>"