--- ./mg.c.orig	2009-09-08 10:41:20.000000000 +0000
+++ ./mg.c	2009-09-08 10:43:29.000000000 +0000
@@ -1543,7 +1543,7 @@ Perl_magic_setisa(pTHX_ SV *sv, MAGIC *m
     stash = GvSTASH(
         SvTYPE(mg->mg_obj) == SVt_PVGV
             ? (GV*)mg->mg_obj
-            : (GV*)SvMAGIC(mg->mg_obj)->mg_obj
+            : (GV*)mg_find(mg->mg_obj, PERL_MAGIC_isa)->mg_obj
     );
 
     mro_isa_changed_in(stash);