nis/yp question about password file

Hello,

I have upgraded some of my machines from 4.x to 5.x and I am having a 
weird problem.

I have

+:::::::::

in my master.passwd file and I am able to login to the machine, but 
after I login the machine seems to think my UID is 100 instead of 501

this problem does not exist in my 4.x machines. when I login, I see my 
uid as it should be in 4.x machines.

can anybody tell what changed in nis/yp that it doesnt work as before 
anymore?

Thanks,
Evren