diff --git a/src/common/String.h b/src/common/String.h
index f23ab600c..d0eee7507 100644
--- a/src/common/String.h
+++ b/src/common/String.h
@@ -509,7 +509,7 @@ public:
 		std::locale const &loc = std::locale::classic();
 		String value(*this);
 		for(value_type &ch : value)
-			if(ch <= std::numeric_limits<ByteString::value_type>::max())
+			if(ch <= static_cast<value_type>(std::numeric_limits<ByteString::value_type>::max()))
 				ch = std::tolower(static_cast<ByteString::value_type>(ch), loc);
 		return value;
 	}
@@ -519,7 +519,7 @@ public:
 		std::locale const &loc = std::locale::classic();
 		String value(*this);
 		for(value_type &ch : value)
-			if(ch <= std::numeric_limits<ByteString::value_type>::max())
+			if(ch <= static_cast<value_type>(std::numeric_limits<ByteString::value_type>::max()))
 				ch = std::toupper(static_cast<ByteString::value_type>(ch), loc);
 		return value;
 	}