Index: boost/archive/iterators/base64_from_binary.hpp
===================================================================
--- boost/archive/iterators/base64_from_binary.hpp	(revision 49167)
+++ boost/archive/iterators/base64_from_binary.hpp	(working copy)
@@ -43,7 +43,7 @@
             "0123456789"
             "+/";
         assert(t < 64);
-        return lookup_table[t];
+        return lookup_table[static_cast<size_t>(t)];
     }
 };