PL SQL Report function showing wrong number or types of arguments in call to '='

function CF_OP_QTYFormula return Number 
is 
  v_qty number(10); 
begin  
   select sum(L_QTY) 
     into v_qty 
     from purchase_01   
    where MONTH_YEAR = :P_MONTH_YEAR -1;   
   return v_qty; 
end;

First of all, I'm pretty sure that MONTH_YEAR =: P_MONTH_YEAR -1; is wrong syntax. The operator =: it's wrong. The right operator is :=.