MySQL Max(isnull(date_field)) or max(coalesce(date_field))

I have a query reading a date from a parent/child relationship and i need to group the records by the max of the date, from whichever tables this comes. My query:

SELECT child.id as id, 
       MAX(pm.start), 
       MAX(membership.start),
       MAX(IFNULL(pm.start,membership.start)) AS start 
FROM   organisation child
LEFT JOIN organisation pmorg ON child.parent_organisation_id = pmorg.id
LEFT JOIN membership ON child.id = membership.organisation_id
LEFT JOIN membership pm ON pmorg.id = pm.organisation开发者_C百科_id

WHERE child.id IN (1,3) AND /*just look at example records*/

    coalesce(pm.start,membership.start) IS NOT NULL AND
    ((membership.active = 1 AND membership.amount_paid > 0.00 ) OR
     (pm.active = 1 AND pm.amount_paid > 0.00))

GROUP BY child.id

My Output is strangely:

1       NULL                    2011-07-01 00:00:00 323031312d30372d30312030303a30303a3030
3       2011-07-01 00:00:00     NULL                323031312d30372d30312030303a30303a3030

I need that last column to be 2011-07-01 and I need both rows. Any idea why max(coalesce()) doesn't work? I'm running MySQL 5.1

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Stupidly this query only breaks in PHPMyAdmin and works completely fine in mysql command line. So no issue here at all!