diff --git a/libcxx/include/locale b/libcxx/include/locale
--- a/libcxx/include/locale
+++ b/libcxx/include/locale
@@ -1476,7 +1476,7 @@
                                                     char const* __len) const
 {
     // Stage 1 - Get number in narrow char
-    char __fmt[6] = {'%', 0};
+    char __fmt[8] = {'%', 0};
     this->__format_int(__fmt+1, __len, is_signed<_Integral>::value, __iob.flags());
     // Worst case is octal, with showbase enabled. Note that octal is always
     // printed as an unsigned value.