company code mismatch

Hi,

We create 10 companycodes and 10 plants .we assigned each company code to each plant.

Ex

ccode plant

0012 - Japan

0101- Uk

But while creating the PO if i create the plant uk with company code 0012 system allowed.it will not show the error message.

if any configuration we want to do

pls suggest.

Regards,

Kshanmugasundaram.