diff --git a/src/c_regex_traits.cpp b/src/c_regex_traits.cpp
index 1b1bcdd9..8a815105 100644
--- a/src/c_regex_traits.cpp
+++ b/src/c_regex_traits.cpp
@@ -514,9 +514,9 @@ void BOOST_REGEX_CALL c_traits_base::do_update_ctype()
       if(std::isxdigit(i))
          class_map[i] |= char_class_xdigit;
    }
-   class_map['_'] |= char_class_underscore;
-   class_map[' '] |= char_class_blank;
-   class_map['\t'] |= char_class_blank;
+   class_map[(unsigned char)'_'] |= char_class_underscore;
+   class_map[(unsigned char)' '] |= char_class_blank;
+   class_map[(unsigned char)'\t'] |= char_class_blank;
    for(i = 0; i < map_size; ++i)
    {
       lower_case_map[i] = (char)std::tolower(i);
diff --git a/src/cpp_regex_traits.cpp b/src/cpp_regex_traits.cpp
index 0c89ad24..00b8c4a4 100644
--- a/src/cpp_regex_traits.cpp
+++ b/src/cpp_regex_traits.cpp
@@ -241,7 +241,7 @@ message_data<char>::message_data(const std::locale& l, const std::string& regex_
 #endif
       for(std::size_t j = 0; j < s.size(); ++j)
       {
-         syntax_map[s[j]] = (unsigned char)(i);
+         syntax_map[(unsigned char)s[j]] = (unsigned char)(i);
       }
    }