enable MTU for vlan on PowerConnect 6248

Hi

For some strange reason I cannot enable MTU for vlans. I get below error:

console(config)#interface vlan 1000

console(config-if-vlan1000)#ip mtu 8000

Could not get the internal interface number associated with the Unit-Slot-Port.

Any idea why?

!System Description "PowerConnect 6248, 3.3.11.2, VxWorks 6.5"
!System Software Version 3.3.11.2