diff --git a/py/modbuiltins.c b/py/modbuiltins.c
index c8e3235f6f..e6f82df6f4 100644
--- a/py/modbuiltins.c
+++ b/py/modbuiltins.c
@@ -137,7 +137,7 @@ MP_DEFINE_CONST_FUN_OBJ_1(mp_builtin_callable_obj, mp_builtin_callable);
 STATIC mp_obj_t mp_builtin_chr(mp_obj_t o_in) {
     #if MICROPY_PY_BUILTINS_STR_UNICODE
     mp_uint_t c = mp_obj_get_int(o_in);
-    char str[4];
+    uint8_t str[4];
     int len = 0;
     if (c < 0x80) {
         *str = c; len = 1;
@@ -159,12 +159,12 @@ STATIC mp_obj_t mp_builtin_chr(mp_obj_t o_in) {
     } else {
         mp_raise_ValueError("chr() arg not in range(0x110000)");
     }
-    return mp_obj_new_str_via_qstr(str, len);
+    return mp_obj_new_str_via_qstr((char*)str, len);
     #else
     mp_int_t ord = mp_obj_get_int(o_in);
     if (0 <= ord && ord <= 0xff) {
-        char str[1] = {ord};
-        return mp_obj_new_str_via_qstr(str, 1);
+        uint8_t str[1] = {ord};
+        return mp_obj_new_str_via_qstr((char*)str, 1);
     } else {
         mp_raise_ValueError("chr() arg not in range(256)");
     }