diff --git a/stl/src/format.cpp b/stl/src/format.cpp
index 67a91356a9d..d639183985a 100644
--- a/stl/src/format.cpp
+++ b/stl/src/format.cpp
@@ -31,7 +31,7 @@ extern "C" [[nodiscard]] __std_win_error __stdcall __std_get_cvt(
             break;
         }
 
-        for (unsigned char _First = _Info.LeadByte[_Idx], _Last = _Info.LeadByte[_Idx + 1]; _First != _Last; ++_First) {
+        for (unsigned int _First = _Info.LeadByte[_Idx], _Last = _Info.LeadByte[_Idx + 1]; _First <= _Last; ++_First) {
             _Pcvt->_Isleadbyte[_First >> 3] |= 1u << (_First & 0b111u);
         }
     }
diff --git a/tests/std/tests/P2286R8_text_formatting_escaping_legacy_text_encoding/test.cpp b/tests/std/tests/P2286R8_text_formatting_escaping_legacy_text_encoding/test.cpp
index b01f72fae10..44ac16c52e4 100644
--- a/tests/std/tests/P2286R8_text_formatting_escaping_legacy_text_encoding/test.cpp
+++ b/tests/std/tests/P2286R8_text_formatting_escaping_legacy_text_encoding/test.cpp
@@ -15,6 +15,8 @@ void test_escaped_string() {
     assert(format("{:?}", "\xEB!") == "\"\\x{eb}!\"");
 
     assert(format("{:?}", "\x81\x40\x40\x81") == "\"\\u{3000}\x40\\x{81}\"");
+
+    assert(format("{:?}", "\xFC\x4B") == "\"\u9ED1\"");
 }
 
 template <class TupleOrPair>