diff --git a/include/internal/catch_message.cpp b/include/internal/catch_message.cpp
index 64c817b7..97983a1e 100644
--- a/include/internal/catch_message.cpp
+++ b/include/internal/catch_message.cpp
@@ -69,10 +69,10 @@ namespace Catch {
 
     Capturer::Capturer( StringRef macroName, SourceLineInfo const& lineInfo, ResultWas::OfType resultType, StringRef names ) {
         auto trimmed = [&] (size_t start, size_t end) {
-            while (names[start] == ',' || isspace(names[start])) {
+            while (names[start] == ',' || isspace(static_cast<unsigned char>(names[start]))) {
                 ++start;
             }
-            while (names[end] == ',' || isspace(names[end])) {
+            while (names[end] == ',' || isspace(static_cast<unsigned char>(names[end]))) {
                 --end;
             }
             return names.substr(start, end - start + 1);