Case causing ORA-01790: expression must have same datatype as corresponding expression error

I'm getting a ORA-01790: expression must have same datatype as corresponding expression error message. Below is the sql I'm using. Is the CASE statement causing this issue? I'm new to sql and this is the first time I've tried using a UNION statement. I've tried a join instead of the UNION but any join I have tried causes the prior_amt field to be blank. Thanks for the help........

SELECT 
    pa.BUSINESS_UNIT as BUS_UNIT, 
    pa.DESCR AS DESCRIPT,
    pdr.DEPTID AS DEPTID, 
    pdr.ASSET_ID AS ASSET_NO, 
    pdr.ACCOUNT_AD AS ACCT_AD, 
    pdr.BOOK AS BOOK,

    MAX(CASE WHEN (pdr.FISCAL_YEAR =2014 AND pdr.ACCOUNTING_PERIOD =11) THEN  pdr.DEPR END) as CURRENT_AMT,
    MAX(CASE WHEN (pdr.FISCAL_YEAR =2104 AND pdr.ACCOUNTING_PERIOD =10) THEN pdr.DEPR  END) as PRIOR_AMT,

    '' AS ACCT_DE,
    '' AS JRNL_ID,
    '' AS JRNL_DT

    FROM PS_ASSET pa

    INNER JOIN PS_DEPR_RPT pdr 
    ON pa.ASSET_ID = pdr.ASSET_ID
    AND pa.BUSINESS_UNIT = pdr.BUSINESS_UNIT

    WHERE 
    pa.BUSINESS_UNIT='A0465'
    AND pdr.BOOK='PERFORM'
    AND ((pdr.FISCAL_YEAR=2014 AND pdr.ACCOUNTING_PERIOD=11) 
    OR (pdr.FISCAL_YEAR=2014 AND pdr.ACCOUNTING_PERIOD=10))

    group by
    pa.business_unit,
    pa.descr,
    pdr.deptid,
    pdr.asset_id,
    pdr.account_ad,
    pdr.book

UNION ALL

    select
    '' as BUS_UNT,
    '' AS DESCRIPT,
    '' AS DEPTID, 
    '' AS ACCT_AD, 
    '' AS BOOK,
    '' AS CURRENT_AMT,
    '' AS PRIOR_AMT,
    pdl.asset_id AS ASSET_NO,
    pdl.account AS ACCT_DE,
    pdl.journal_id AS JRNL_ID,
    pdl.journal_date AS JRNL_DT

    from ps_dist_ln pdl

    where
    book = 'PERFORM'
    and business_unit = 'A0465'
    and fiscal_year = 2014
    and accounting_period = 11
    and distribution_type = 'DE'

Your problem is that you've put the columns in a different order in each half of the union. The columns have to match up, in the same order, between the two halves. It's not to do with the CASE expression.

Also, where you've written 2104, it should probably be 2014.