diff -up perl-5.10.0/mg.c.BAD perl-5.10.0/mg.c
--- perl-5.10.0/mg.c.BAD	2009-03-11 13:10:22.000000000 -0400
+++ perl-5.10.0/mg.c	2009-03-11 13:08:54.000000000 -0400
@@ -1543,10 +1543,11 @@ 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);
+    if (stash)
+        mro_isa_changed_in(stash);
 
     return 0;
 }
--- a/t/mro/pkg_gen.t
+++ b/t/mro/pkg_gen.t
@@ -4,7 +4,7 @@ use strict;
 use warnings;
 
 chdir 't' if -d 't';
-require q(./test.pl); plan(tests => 6);
+require q(./test.pl); plan(tests => 7);
 
 {
     package Foo;
@@ -34,3 +34,7 @@ is(mro::get_pkg_gen('Foo'), 1, "pkg_gen 1 for undef %Pkg::");
 
 delete $::{"Foo::"};
 is(mro::get_pkg_gen('Foo'), 0, 'pkg_gen 0 for delete $::{Pkg::}');
+
+delete $::{"Quux::"};
+push @Quux::ISA, "Woot"; # should not segfault
+ok(1, "No segfault on modification of ISA in a deleted stash");