SQL & PL/SQL

Created trigger to check the sum of two values.
the sum gets inserted to attribute i declared.
this sum should then get inserted on another table
where the id = :newid
if the sum was less than or equal to 0 then error message.

Creates fine.

insert into runway values
(
'12',runway_seq.nextval,'21-aug-06','22-aug-06',
)
;

When i insert information into the the table to run this trigger, i get

ORA-00937: not a single-group group function
ORA-06512: - the line of code which i know works before the this trigger was implemented!
ORA-04088: - the trigger name

Heard about grouping but does no difference and where would i put it

Any ideas??