Running Totals in BI Publisher + Cannot convert to number

Over on the BI Publisher forums, I spotted a interesting one error when attempting to put page totals using a running total using updateable variables and templated footers:

Once I run the RTF am getting error "Caused by: oracle.xdo.parser.v2.XPathException: Cannot convert to number"

As per the example in document XML Publisher Users Guide 11i the syntax seemed right, i.e. where INVAMT is the field to calculate running total and RTotalVar is the running total variable:

Initialize variable:

<?xdoxslt:set_variable($_XDOCTX, 'RTotalVar', 0)?>

Set variable:

<?xdoxslt:set_variable($_XDOCTX, 'RTotalVar', xdoxslt:get_variable($_XDOCTX,'RTotalVar') + INVAMT)?> 

Show variable:

<?xdoxslt:get_variable($_XDOCTX, 'RTotalVar')?>

After a bit of tinkering with the RTF and XML, I realized the syntax was correct, so what was going on?

One extenuating factor was the location of the "set variable" form field inside a nested table. Time to move it out, no need for it to be there - unless the grouping mattered ... in this case it didn't. So moving the "set variable" form field out of the table fixed things. Moral of the story?

If somethin's not working and you think it should, and the error output ain't doin it for ya - simplify simplify - or should that be KISS KISS - until it works ;-)