diff -r e5d79e6deeb5 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Fri Jan 30 23:35:03 2015 +0200
+++ b/Objects/unicodeobject.c	Sat Jan 31 00:18:45 2015 +0200
@@ -738,6 +738,8 @@ PyUnicode_FromFormatV(const char *format
              f++;
              while (*f && *f != '%' && !isalpha((unsigned)*f))
                  f++;
+             if (!*f)
+                 break;
              if (*f == 's' || *f=='S' || *f=='R')
                  ++callcount;
          }