Display Sum of time in HH:MM format in SSRS 2008

I have a Dataset with values and displaying total_service_time in HH:MM format from my query

Query :

(SELECT    CAST(total / 60 AS varchar(8)) + ':' + CAST(total % 60 AS varchar(2)) AS Expr1
 FROM (SELECT CAST(SUM(Action.[Travel Time] + Action.[Total Productive Time]) AS int) AS total) AS T) AS total_service_time                     

I am getting #Error while adding expression =Sum(Fields!total_service_time.Value) in last row to get the sum of total_service_time in HH:MM format.

Is there any way to get the Sum of time in HH:MM format in last row

Error Screen:

Thanks,

Salman