WITH a AS
(
    SELECT 
        ii.RPTCategory AS Cat,
        YEAR(ii.CreateDate) AS CatYear, MONTH(ii.CreateDate) AS CatMonth,
        COUNT(*) AS CatNo
    FROM 
        InsInvoice ii 
    WHERE
        ii.RPTCategory IN ('12')
    GROUP BY
        ii.RPTCategory, YEAR(ii.CreateDate), MONTH(ii.CreateDate)
),
b AS
(
    SELECT 
        ii.Category AS Cat,
        YEAR(ii.CreateDate) AS CatYear, MONTH(ii.CreateDate) AS CatMonth,
        COUNT(*) AS CatNo
    FROM 
        InsInvoice ii 
    WHERE
        ii.Category IN ('41')
    GROUP BY
        ii.Category, YEAR(ii.CreateDate), MONTH(ii.CreateDate)
)
SELECT *
FROM 
    (a UNION b)

I get the error

Incorrect syntax near the keyword union

All searches I've done, the user encounters the error when using order by within their sub select statements but I'm not doing that. I've tried splitting this out into sub select statements instead of using the with clause but I get the same error.