Testing For Null In Crosstab

I'm sure that I'm being stupid but this is driving me nuts. The case statements in the following query always return the ELSE and never return the THEN for the NULL.

Code:

select day(ums_users.created), count(*) as Total,
SUM(CASE ums_user_info.info_key WHEN NULL THEN 1 ELSE 0 END) AS UnReg,
SUM(CASE ums_user_info.info_key WHEN NULL THEN 0 ELSE 1 END) AS Reg
from ums_users
left join ums_user_map
on ums_users.user_id = ums_user_map.user_id
left join ums_groups
on ums_user_map.group_id = ums_groups.group_id
left join ums_user_info
on ums_user_info.user_id = ums_users.user_id

where (ums_groups.group_id = '43A73FAE-0C2C-4ED8-BE1E-B32C12DB163D'
or ums_groups.group_parent_id = '43A73FAE-0C2C-4ED8-BE1E-B32C12DB163D')
AND year(ums_users.created) = '2005' and month(ums_users.created) = '01'
GROUP BY day(ums_users.created)
ORDER BY day(ums_users.created)


I'd be really grateful if somebody could explain what I'm doing wrong.

TIA

Nick