--- string_format.complete.h    2010-06-25 15:21:20.458257318 -0300
+++ string_format.crippled.h    2010-06-25 15:24:46.264227295 -0300
@@ -454,6 +454,10 @@
     if (*first_idx_is_an_integer == 0 && PyErr_Occurred())
         return 0;

+    /* Don't allow negative index fields */
+    if (*first_idx_is_an_integer && *first_idx < 0)
+        *first_idx_is_an_integer = 0;
+
     field_name_is_empty = first->ptr >= first->end;

     /* If the field name is omitted or if we have a numeric index