Index: boost/lexical_cast.hpp
===================================================================
--- boost/lexical_cast.hpp	(revision 76047)
+++ boost/lexical_cast.hpp	(working copy)
@@ -863,7 +863,7 @@
             CharT const thousands_sep = grouping_size ? np.thousands_sep() : 0;
             CharT const decimal_point = np.decimal_point();
             bool found_grouping = false;
-            unsigned int last_grouping_pos = grouping_size - 1;
+            std::string::size_type last_grouping_pos = grouping_size - 1;
 #else
             CharT const decimal_point = lcast_char_constants<CharT>::c_decimal_separator;
 #endif