diff --git a/include/cxxopts.hpp b/include/cxxopts.hpp
index 76dcc1a..ee02c57 100644
--- a/include/cxxopts.hpp
+++ b/include/cxxopts.hpp
@@ -2090,12 +2090,12 @@ format_description
   {
     appendNewLine = false;
 
-    if (std::isblank(*previous))
+    if (std::isblank(*previous, std::locale::classic()))
     {
       lastSpace = current;
     }
 
-    if (!std::isblank(*current))
+    if (!std::isblank(*current, std::locale::classic()))
     {
       onlyWhiteSpace = false;
     }