Cannot expand the group when a custom field contains decimal figures (2012 SP1)

Symptoms:
In a report that uses grouping there is a custom field calculating a percentage. When trying to expand a grouped line where the group header contains "0.00", the group does not expand.

Reason:
Custom field does not handle NULL values, causing certain lines to be excluded from the result set.

Resolution:
Use a CASE statement to handle NULL values, and round the calculated values to 2 decimals.

Example:
CASE
WHEN ( ISNULL([SomeField], 0) ) = 0
THEN 0
ELSE
(
ROUND((ISNULL([SomeField], 0 ) * 100) / [SomeOtherField] ,2)
)