diff --git a/src/c_regex_traits.cpp b/src/c_regex_traits.cpp
index 8baa06b2..494fe65e 100644
--- a/src/c_regex_traits.cpp
+++ b/src/c_regex_traits.cpp
@@ -499,7 +499,7 @@ void BOOST_REGEX_CALL c_traits_base::do_update_ctype()
 
    // now update the character class map,
    // and lower case map:
-   std::memset(class_map, 0, map_size);
+   std::memset(class_map, 0, sizeof(class_map));
    for(i = 0; i < map_size; ++i)
    {
       if(std::isalpha(i))
@@ -519,9 +519,9 @@ 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/w32_regex_traits.cpp b/src/w32_regex_traits.cpp
index bcd888d1..ddb8d20e 100644
--- a/src/w32_regex_traits.cpp
+++ b/src/w32_regex_traits.cpp
@@ -302,7 +302,7 @@ void BOOST_REGEX_CALL w32_traits_base::do_init()
       {
          class_map[i] &= char_class_win;
       }
-      class_map['_'] |= char_class_underscore;
+      class_map[(unsigned char)'_'] |= char_class_underscore;
       LCMapStringA(GetUserDefaultLCID(), LCMAP_LOWERCASE, buf, map_size, lower_case_map, map_size);
       //
       // update our collating elements: