diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index e2f1ed3..3cf7fa6 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -974,7 +974,7 @@
     while (s < e) {
         Py_UNICODE ch;
         restart:
-        ch = *s;
+        ch = (unsigned char) *s;
 
         if (inShift) {
             if ((ch == '-') || !B64CHAR(ch)) {