diff --git a/locale.c b/locale.c
index db0fc6cb8dfd..cbde262410f7 100644
--- a/locale.c
+++ b/locale.c
@@ -2244,6 +2244,11 @@ S_bool_setlocale_2008_i(pTHX_
 
     int mask = category_masks[index];
     const locale_t entry_obj = uselocale((locale_t) 0);
+
+#  ifdef MULTIPLICITY
+    assert(entry_obj== LC_GLOBAL_LOCALE || entry_obj == PL_cur_locale_obj);
+#  endif
+
     const char * locale_on_entry = querylocale_i(index);
 
     DEBUG_Lv(PerlIO_printf(Perl_debug_log,