diff --git a/include/internal/catch_test_case_info.hpp b/include/internal/catch_test_case_info.hpp
index 90f53411..57963ff8 100644
--- a/include/internal/catch_test_case_info.hpp
+++ b/include/internal/catch_test_case_info.hpp
@@ -30,7 +30,7 @@ namespace Catch {
             return TestCaseInfo::None;
     }
     inline bool isReservedTag( std::string const& tag ) {
-        return parseSpecialTag( tag ) == TestCaseInfo::None && tag.size() > 0 && !isalnum( tag[0] );
+        return parseSpecialTag( tag ) == TestCaseInfo::None && tag.size() > 0 && !isalnum( static_cast<unsigned char>(tag[0]) );
     }
     inline void enforceNotReservedTag( std::string const& tag, SourceLineInfo const& _lineInfo ) {
         if( isReservedTag( tag ) ) {