--- /usr/src/contrib/libc++/include/locale	(revision 296556)
+++ /usr/src/contrib/libc++/include/locale	(working copy)
@@ -1645,7 +1645,7 @@ 
     this->__format_int(__fmt+1, __len, false, __iob.flags());
     const unsigned __nbuf = (numeric_limits<unsigned long long>::digits / 3)
                           + ((numeric_limits<unsigned long long>::digits % 3) != 0)
-                          + 1;
+                          + (__fmt[1] == '#') + 1;
     char __nar[__nbuf];
 #ifdef _LIBCPP_LOCALE__L_EXTENSIONS
     int __nc = snprintf_l(__nar, sizeof(__nar), _LIBCPP_GET_C_LOCALE, __fmt, __v);