full outer join?

Question

hi,
      please give answer

CREATE PROCEDURE Prcexpincview

(

  @month   varchar(50),

  @year    varchar(50)

)

as

SELECT    tblexpense.expname,tblexpense.expamount,tblpayments.vpayment 

FROM         dnalabsoft.tblexpense FULL OUTER JOIN

                      dnalabsoft.tblpayments ON dnalabsoft.tblexpense.ipaymentid = dnalabsoft.tblpayments.ipaymentid

                      where month(dbilldate)=@month and  year(dbilldate)=@year group by expname 

Error Occured

Msg 209, Level 16, State 1, Procedure Prcexpincview, Line 11

Ambiguous column name 'dbilldate'.

Msg 209, Level 16, State 1, Procedure Prcexpincview, Line 11

Ambiguous column name 'dbilldate'.