diff -up perl-5.10.0/mg.c.crr perl-5.10.0/mg.c
--- perl-5.10.0/mg.c.crr	2007-12-18 11:47:08.000000000 +0100
+++ perl-5.10.0/mg.c	2008-05-26 15:28:17.000000000 +0200
@@ -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);