Union causing Firebird 2.5 query to fail - sql

Question

This SQL (Firebird 2.5, Dialect 1) query works fine when I run it in Database Workbench, but it fails when I run it under IBO Console with:
count of column list and variable list do not match.
These two queries run successfully if I run them alone under IBO console, so I'm inferring that the problem is because of the "Union." The number and types of columns match for the two queries--both return string, smallint, string (IBO console regards this as a memo), string--so Union should be valid (and DB Workbench finds it so). Thinking that maybe the length of the data was different, I tried casting the third column as a VarChar(500) but that didn't help. Looking for ideas as to why this doesn't work since the part of our app that is executing the query is apparently choking on this in the same way that IBO Console is.
Select QBI.TXNID as ID,
Cast (1 as SmallInt) as TransactionType,
("Invoice " || QBI.REFNUMBER || ": $" || round(QBI.SUBTOTAL, 2) || " on " || QBI.TXNDATE || " for " || QBI.CUSTOMERREF_FULLNAME) as description,
case
when (QBI.CLASSREF_LISTID = "") then "Invoice has no class in Quickbooks"
else "Invoice class doesn't match any dept on job " || JA.JOBID
end as Problem
from QBINVOICE QBI
Join JOBACCOUNTINGID JA
on QBI.CUSTOMERREF_LISTID = JA.jobaccountingid
and QBI.SOURCEID = JA.SOURCEID
left Join CHARTOFACCOUNTS CA
on (CA.qblistid = QBI.CLASSREF_LISTID and CA.qbsourceID = QBI.SOURCEID)
and CA.CHARTACCOUNTTYPE = "SYSTEM"
and CA.CHARTFETCH = "Y"
left Join DEPARTMENTJOB DJ
on JA.JOBID = DJ.JobID
and DJ.departmentID = CA.DEPARTMENTID
where DJ.DEPARTMENTID is null
and QBI.TXNDATE >= "02/01/2017"
union all
select
QBELD.TXNLINEID as ID,
Cast (2 as SmallInt) as TransactionType,
QBB.VENDORREF_FULLNAME || " bill on " || QBB.TXNDATE || ": $" || round(QBELD.AMOUNT, 2) || " " || QBELD.ACCOUNTREF_FULLNAME || " expense" as description,
case
when (QBELD.CLASSREF_LISTID = "") then "Expense has no class in Quickbooks"
else "Expense class doesn't match any dept on job " || JA.JOBID
end as Problem
from QBTxnExpenseLineDetail QBELD
JOIN QBBILL QBB
on QBELD.TXNLINEID = QBB.TXNID
and QBELD.SOURCEID = QBB.SOURCEID
Join JOBACCOUNTINGID JA
on QBELD.CUSTOMERREF_LISTID = JA.jobaccountingid
and QBELD.SOURCEID = JA.SOURCEID
left Join CHARTOFACCOUNTS CA
on (CA.qblistid = QBELD.CLASSREF_LISTID and CA.qbsourceID = QBELD.SOURCEID)
and CA.CHARTACCOUNTTYPE = "SYSTEM"
and CA.CHARTFETCH = "Y"
left Join DEPARTMENTJOB DJ
on JA.JOBID = DJ.JobID
and DJ.departmentID = CA.DEPARTMENTID
where DJ.DEPARTMENTID is null
and QBB.TXNDATE >= "02/01/2017"