problem with DAYOFWEEK_ISO in an SQLRPGLE program

Hi everyone
We are on V7r3
I am trying to determine the first business day of the following month
I have a simple program to start of with It takes a date, calculates the first of the following month, and then finds the day of the week (ISO)
The code is below (like I said - its simple)
Using debug, I can see that HldDayOfWeek_ISO from DayOfWeek_ISO(Beginning_of_Next_Month) is zero (which is NOT a correct value)
But HldDayOfWeek_ISO2 from DAYOFWEEK_ISO(CURRENT_DATE) is 3 - which is correct - today being Wednesday
***NOTE*** - The value of Beginning_Of_Next_month is '2019-02-01' , according to debug
Can anyone see what is wrong with this code?
Why I am not getting the DAYOFWEEK_ISO for Beginning_Of_Next_Month?
As always, all answers replies gratefully accepted

dcl-s MyDate date inz(d'2019-01-02');
dcl-s Beginning_of_Next_Month date;
dcl-s HldDayOfWeek_ISO zoned(2:0);
dcl-s HldDayOfWeek_ISO2 zoned(2:0);

//-------------------------------------------------------------------
// Stand Alone Fields - BOTTOM
//-------------------------------------------------------------------
exec Sql Set Option Commit=*NC, CLOSQLCSR=*ENDMOD, DECMPT=*PERIOD;
exec Sql Set Option DATFMT = *ISO, NAMING = *SQL;

Beginning_of_Next_Month = MyDate - %Days(%subdt( myDate:*days)-1) +
%months(1) ;
exec sql
select DayOfWeek_ISO(Beginning_of_Next_Month)
into :HldDayOfWeek_ISO
FROM SYSIBM.SYSDUMMY1;

exec sql
SELECT DAYOFWEEK_ISO(CURRENT_DATE)
INTO :HldDayOfWeek_ISO2 FROM SYSIBM.SYSDUMMY1;

*inlr = '1';
return;

Alan Shore
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