diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index a62e929..16d97c3 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -1523,7 +1523,7 @@
     while (s < e) {
         Py_UNICODE ch;
         restart:
-        ch = *s;
+        ch = (unsigned char) *s;
 
         if (inShift) {
             if ((ch == '-') || !B64CHAR(ch)) {