Why serial of ram is not shown in system information?

My macbook is the late 2013. When i check the information of ram, serial number is not shown. What s the reason?

BANK 0/DIMM0:

Size: 4 GB

Type: DDR3

Speed: 1600 MHz

Status: OK

Manufacturer: 0x02FE

Part Number: 0x000000000000000000000000000000000000

Serial Number: -

BANK 1/DIMM0:

Size: 4 GB

Type: DDR3

Speed: 1600 MHz

Status: OK

Manufacturer: 0x02FE

Part Number: 0x000000000000000000000000000000000000

Serial Number: -