diff --git a/src/catch2/internal/catch_clara_upstream.hpp b/src/catch2/internal/catch_clara_upstream.hpp
index d77f30c4..64b762d0 100644
--- a/src/catch2/internal/catch_clara_upstream.hpp
+++ b/src/catch2/internal/catch_clara_upstream.hpp
@@ -667,7 +667,7 @@ namespace detail {
     }
     inline auto convertInto( std::string const &source, bool &target ) -> ParserResult {
         std::string srcLC = source;
-        std::transform( srcLC.begin(), srcLC.end(), srcLC.begin(), []( char c ) { return static_cast<char>( std::tolower(c) ); } );
+        std::transform( srcLC.begin(), srcLC.end(), srcLC.begin(), []( unsigned char c ) { return static_cast<char>( std::tolower(c) ); } );
         if (srcLC == "y" || srcLC == "1" || srcLC == "true" || srcLC == "yes" || srcLC == "on")
             target = true;
         else if (srcLC == "n" || srcLC == "0" || srcLC == "false" || srcLC == "no" || srcLC == "off")
diff --git a/src/catch2/internal/catch_string_manip.cpp b/src/catch2/internal/catch_string_manip.cpp
index b24ec880..1d75b2e5 100644
--- a/src/catch2/internal/catch_string_manip.cpp
+++ b/src/catch2/internal/catch_string_manip.cpp
@@ -18,7 +18,7 @@ namespace Catch {
 
     namespace {
         char toLowerCh(char c) {
-            return static_cast<char>( std::tolower( c ) );
+            return static_cast<char>( std::tolower( static_cast<unsigned char>(c) ) );
         }
     }
 
diff --git a/third_party/clara.hpp b/third_party/clara.hpp
index 9f4f1b2c..eb4c7275 100644
--- a/third_party/clara.hpp
+++ b/third_party/clara.hpp
@@ -667,7 +667,7 @@ namespace detail {
     }
     inline auto convertInto( std::string const &source, bool &target ) -> ParserResult {
         std::string srcLC = source;
-        std::transform( srcLC.begin(), srcLC.end(), srcLC.begin(), []( char c ) { return static_cast<char>( ::tolower(c) ); } );
+        std::transform( srcLC.begin(), srcLC.end(), srcLC.begin(), []( unsigned char c ) { return static_cast<char>( ::tolower( c ) ); } );
         if (srcLC == "y" || srcLC == "1" || srcLC == "true" || srcLC == "yes" || srcLC == "on")
             target = true;
         else if (srcLC == "n" || srcLC == "0" || srcLC == "false" || srcLC == "no" || srcLC == "off")