'Assertion ((svtype)((sv)->sv_flags & 0xff)) != 0xff failed: f...'

##</code><code>##

/* Sadly there are some parts of the core that have pointers to already-freed
   SV heads, and rely on being able to tell that they are now free. So mark
   them all by using a consistent macro.  */
#define SvIS_FREED(sv)  ((sv)->sv_flags == SVTYPEMASK)

##</code><code>##

/* delete a back-reference to ourselves from the backref magic associated
 * with the SV we point to.
 */

STATIC void
S_sv_del_backref(pTHX_ SV *tsv, SV *sv)
{
...
    assert(!SvIS_FREED(av));