diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 1e3b812..91e7524 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -738,6 +738,8 @@
              f++;
              while (*f && *f != '%' && !isalpha((unsigned)*f))
                  f++;
+             if (!*f)
+                 break;
              if (*f == 's' || *f=='S' || *f=='R')
                  ++callcount;
          }