I had something similiar happen.
Try replacing all the OLDVAL( "field", "table")
with NVL( OLDVAL( "field", "table"), 0 ).
Substitute an empty field value for 0, if the field is not numeric.
Just a guess is that during the SQL, FoxPro create a temporary index.
And THAT does not accept NULLs.
- Andy Rice
San Diego, CA