diff --git a/src/logic/scripting/lua/libs/libutf8.cpp b/src/logic/scripting/lua/libs/libutf8.cpp
index 163f07e4e..918bda095 100644
--- a/src/logic/scripting/lua/libs/libutf8.cpp
+++ b/src/logic/scripting/lua/libs/libutf8.cpp
@@ -66,19 +66,13 @@ static int l_sub(lua::State* L) {
 }
 
 static int l_upper(lua::State* L) {
-    auto string = util::str2u32str_utf8(lua::require_string(L, 1));
-    for (auto& c : string) {
-        c = std::towupper(c);
-    }
-    return lua::pushstring(L, util::u32str2str_utf8(string));
+    auto string = util::str2wstr_utf8(lua::require_string(L, 1));
+    return lua::pushwstring(L, util::upper_case(string));
 }
 
 static int l_lower(lua::State* L) {
-    auto string = util::str2u32str_utf8(lua::require_string(L, 1));
-    for (auto& c : string) {
-        c = std::towlower(c);
-    }
-    return lua::pushstring(L, util::u32str2str_utf8(string));
+    auto string = util::str2wstr_utf8(lua::require_string(L, 1));
+    return lua::pushwstring(L, util::lower_case(string));
 }
 
 static int l_encode(lua::State* L) {
diff --git a/src/util/stringutil.cpp b/src/util/stringutil.cpp
index 0329305c3..f97eed83e 100644
--- a/src/util/stringutil.cpp
+++ b/src/util/stringutil.cpp
@@ -507,22 +507,32 @@ double util::parse_double(const std::string& str, size_t offset, size_t len) {
     return parse_double(str.substr(offset, len));
 }
 
-std::wstring util::lower_case(const std::wstring& str) {
-    std::wstring result = str;
+template<typename CharT>
+static std::basic_string<CharT> lower_case(const std::basic_string<CharT>& str) {
+    std::basic_string<CharT> result = str;
     for (uint i = 0; i < result.length(); i++) {
-        result[i] = static_cast<wchar_t>(std::tolower(str[i], locale));
+        result[i] = std::tolower(str[i], locale);
     }
     return result;
 }
 
-std::wstring util::upper_case(const std::wstring& str) {
-    std::wstring result = str;
+template<typename CharT>
+static std::basic_string<CharT> upper_case(const std::basic_string<CharT>& str) {
+    std::basic_string<CharT> result = str;
     for (uint i = 0; i < result.length(); i++) {
-        result[i] = static_cast<wchar_t>(std::toupper(str[i], locale));
+        result[i] = std::toupper(str[i], locale);
     }
     return result;
 }
 
+std::wstring util::lower_case(const std::wstring& str) {
+    return ::lower_case(str);
+}
+
+std::wstring util::upper_case(const std::wstring& str) {
+    return ::upper_case(str);
+}
+
 std::wstring util::capitalized(const std::wstring& str) {
     if (str.empty()) return str;
     return std::wstring({static_cast<wchar_t>(std::toupper(str[0], locale))}) +