diff --git a/t/loc_tools.pl b/t/loc_tools.pl
index 0fcffdcd48c0..3017c90fefb9 100644
--- a/t/loc_tools.pl
+++ b/t/loc_tools.pl
@@ -187,6 +187,11 @@ ($$$$)
             return;
         }
 
+        # Commas in locale names are bad in Windows, and there is a bug in
+        # some versions where setlocale() turns a legal input locale name into
+        # an illegal return value, which it can't later parse.
+        return if $result =~ /,/;
+
         return unless $plays_well || $allow_incompatible;
     }