diff --git a/locale.c b/locale.c
index 39e55379cf9b..2866778c16df 100644
--- a/locale.c
+++ b/locale.c
@@ -2062,7 +2062,9 @@ Perl_init_i18nl10n(pTHX_ int printwarn)
 
                 trial_locale = system_default_locale;
             }
-#    endif /* WIN32 */
+#    else
+#      error SYSTEM_DEFAULT_LOCALE only implemented for Win32
+#    endif
 #  endif /* SYSTEM_DEFAULT_LOCALE */
 
         }   /* For i > 0 */