SQL & PL/SQL

I have a procedure with many case statements in it that compare data within a table and populate an error description of the dates don't validate. (small code snip below)

create or replace procedure "PROC_TEMP_UPDATE"
is
begin
update temp_update
set error_desc = 
    CASE
    WHEN (error_desc != 'MATCH')
        THEN 'PCFN/TCN combo not found in destination table.'
    WHEN (error_desc = 'SKIP')
        THEN NULL
    WHEN (ge_ata is null and az_ata is not null)
        THEN 'Enter GE ATA Date before entering AZ ATA Date.'
    WHEN ((ge_ata is not null and az_ata is not null) and ge_ata > az_ata)
        THEN 'GE ATA Date cannot be greater than AZ ATA Date.'
       end proc_temp_update;

Instead of having to enter the bottom 2 WHEN statements for every date I created these functions:
(Function 1: check if Date1 is null and Date2 is not.
Function 2:if both dates are not null, check to see if Date1 is Greater than Date2.)

function Is_D1_NULL
            (Date1  DATE, Date2 DATE)
            return BOOLEAN is

      result     BOOLEAN := FALSE; 
   begin 
      if (Date1 is null and Date2 is not null) then  
         result := TRUE; 
      end if; 
      return (result); 
   end Is_D1_NULL;

  
 function D1_Greater_D2
            (Date1 DATE
             Date2 DATE)
            return BOOLEAN is

      result    BOOLEAN := FALSE;   
    begin   
        if ((Date1 is not null and Date2 is not null) and Date1 > Date2) then
            result := TRUE;
        end if;     
        return (result);    
    end D1_Greater_D2;

But when I change the WHEN statements to:

WHEN Is_D1_NULL(ge_ata,az_ata)
    THEN 'Enter GE ATA Date before entering AZ ATA Date.'      

AND

WHEN D1_Greater_D2(ge_ata,az_ata)   
    THEN 'GE ATA Date cannot be greater than AZ ATA Date.'

The procedure doesn't compile anymore.

Is there something wrong with the code? or is this not possible?

Thanks
Steven