diff --git a/luprex/core/cpp/util.cpp b/luprex/core/cpp/util.cpp
index ccb3df7a..dc872e79 100644
--- a/luprex/core/cpp/util.cpp
+++ b/luprex/core/cpp/util.cpp
@@ -313,12 +313,13 @@ void quote_string(const eng::string &s, std::ostream *os) {
                 (*os) << c;
             }
         } else {
+            unsigned int value = ((unsigned char)c);
             switch (c) {
             case '\n': (*os) << "\\n"; break;
             case '\t': (*os) << "\\t"; break;
             case '\r': (*os) << "\\r"; break;
             default:
-                (*os) << "\\" << std::setfill('0') << std::setw(3) << int(c);
+                (*os) << "\\" << std::setfill('0') << std::setw(3) << value;
                 break;
             }
         }