Fwd: Segmentation fault on RelationGetDescr in my first extension

I use check_password_hook and my function executes at the moment of changing user password.

But if I try to open a table inside this function, I get Segmentation fault on the line with   "RelationGetDescr".

What am I doing wrong?

void
_PG_init(void)
{
prev_check_password_hook = check_password_hook;
check_password_hook = check_password;
}

...

void check_password(const char *username, const char *shadow_pass, PasswordType password_type, Datum validuntil_time, bool validuntil_null)
{
Relation rel;
TupleDesc tupdesc;
int Natts;

Oid tbl_oid = DatumGetObjectId(DirectFunctionCall1(to_regclass, CStringGetTextDatum("my_test_table")));
if (OidIsValid(tbl_oid)) {
rel = table_open(tbl_oid, RowExclusiveLock);
tupdesc = RelationGetDescr(rel); // !!! server process (PID 70525) was terminated by signal 11: Segmentation fault
Natts = tupdesc->natts;
table_close(rel, RowExclusiveLock);
} else {
ereport(NOTICE, (errmsg("IndexRelationId (%d) is NOT valid !", tbl_oid)));
}
}