diff --git a/stl/src/format.cpp b/stl/src/format.cpp
index 67a91356a9..96527ed68d 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 (int _First = _Info.LeadByte[_Idx], _Last = _Info.LeadByte[_Idx + 1]; _First <= _Last; ++_First) {
             _Pcvt->_Isleadbyte[_First >> 3] |= 1u << (_First & 0b111u);
         }
     }