diff --git a/include/internal/catch_test_case_info.cpp b/include/internal/catch_test_case_info.cpp
index 9bf68912..46164221 100644
--- a/include/internal/catch_test_case_info.cpp
+++ b/include/internal/catch_test_case_info.cpp
@@ -37,7 +37,7 @@ namespace Catch {
             return TestCaseInfo::None;
     }
     bool isReservedTag( std::string const& tag ) {
-        return parseSpecialTag( tag ) == TestCaseInfo::None && tag.size() > 0 && !std::isalnum( tag[0] );
+        return parseSpecialTag( tag ) == TestCaseInfo::None && tag.size() > 0 && !std::isalnum( static_cast<unsigned char>(tag[0]) );
     }
     void enforceNotReservedTag( std::string const& tag, SourceLineInfo const& _lineInfo ) {
         CATCH_ENFORCE( !isReservedTag(tag),