using where clause in MySql return "Unknown column xxxx in 'where clause'

Cause : 

$this->db->query("
SELECT d.nm_pt, d.kd_pt  , a.status, count(a.status) 
from tbl_mahasiswa_transaksi a 
JOIN tbl_mahasiswa b ON a.nim =  b.nim 
JOIN tbl_pendaftar c on b.no_pendaftaran = c.no_pendaftaran 
JOIN tbl_pt d ON d.kd_pt = c.kd_pt 
WHERE d.kd_pt = $kode_pt
GROUP BY a.status,kd_pt

");

Answer :

'$kode_pt'