diff --git a/luprex/cpp/drv/drvutil.cpp b/luprex/cpp/drv/drvutil.cpp
index e25d039b..c4bbf273 100644
--- a/luprex/cpp/drv/drvutil.cpp
+++ b/luprex/cpp/drv/drvutil.cpp
@@ -242,7 +242,7 @@ static void sbwrite_uint64(std::ostream *s, uint64_t v) {
 // This encoding can be read by StreamBuffer::read_string.
 //
 static void sbwrite_string(std::ostream *s, std::string_view sv) {
-    s->put(0xFF);
+    s->put('\xFF');
     sbwrite_uint64(s, sv.size());
     s->write(sv.data(), sv.size());
 }
@@ -250,7 +250,7 @@ static void sbwrite_string(std::ostream *s, std::string_view sv) {
 // This encoding can be read by StreamBuffer::read_string.
 //
 static bool sbwrite_file(std::ostream *s, const std::filesystem::path &fn) {
-    s->put(0xFF);
+    s->put('\xFF');
     uint64_t pos1 = s->tellp();
     sbwrite_uint64(s, 0);
     uint64_t pos2 = s->tellp();