Application Express :: Error When Refreshing Report - Widget Failure?

I am trying to refresh a report with dynamic action. And getting the following errors:

{"dialog":{"uv":true,"row":[{"V":"Widget Failure
ORA-20876: Stop APEX Engine,
classic_report"}]}}

I have identified the problematic part. I have a multi-select select list and my query goes like this:

select o.order_id, s.salesrep_name
from ml_orders o, ml_order_salesrep_mapping m
where m.order_id(+) = o.order_Id
and case when :P22_SALESREP is null then 1

[code]...