Union query errors in Interbase

[Nick Alexander]

I have 2 queries that I want to union. I can run each query
separately and I get a result. If I run both of them with a union, I
get the error message:

Invalid token.
Dynamic SQL Error.
SQL error code = -104.
Invalid command.
Data type unknown.

The full union query is:

SELECT JOB_COST_HEADER.PROJ_MASTER_REC_ID,
JOB_COST_ITEM.GLOB_PHASE_REC_ID, JOB_COST_ITEM.NAME,
Sum(JOB_COST_ITEM.QUANTITY) AS BudgetQuantity,
Sum(JOB_COST_ITEM.QUANTITY*JOB_COST_ITEM.COST_PRICE) AS BudgetCost,
Sum(0) AS PurchaseQuantity,
Sum(0) AS PurchaseCostPrice
FROM JOB_COST_ITEM INNER JOIN JOB_COST_HEADER ON
JOB_COST_ITEM.JOB_COST_HEADER_REC_ID = JOB_COST_HEADER.REC_ID
WHERE JOB_COST_HEADER.IS_CURRENT='Y' AND JOB_COST_HEADER.HEADER_TYPE=0
GROUP BY JOB_COST_HEADER.PROJ_MASTER_REC_ID,
JOB_COST_ITEM.GLOB_PHASE_REC_ID, JOB_COST_ITEM.NAME

union

SELECT PURCHASE_HEADER.PROJ_MASTER_REC_ID,
PURCHASE_ITEM.GLOB_PHASE_REC_ID, PURCHASE_ITEM.NAME,
Sum(0) AS BudgetQuantity,
Sum(0) AS BudgetCost,
Sum(PURCHASE_ITEM.QUANTITY) AS PurchaseQuantity,
Sum(PURCHASE_ITEM.QUANTITY*PURCHASE_ITEM.COST_PRICE) AS
PurchaseCostPrice
FROM PURCHASE_ITEM INNER JOIN PURCHASE_HEADER ON
PURCHASE_ITEM.PURCHASE_HEADER_REC_ID = PURCHASE_HEADER.REC_ID
GROUP BY PURCHASE_HEADER.PROJ_MASTER_REC_ID,
PURCHASE_ITEM.GLOB_PHASE_REC_ID, PURCHASE_ITEM.NAME

Can anyone shed any light?