derived column cannot convert between unicode and non-unicode string data types

i have a problem with my data flow

my flow is ole db data source-->derived column-->ole db destrination

at the derived column trasformation a add a new column called "entity_code"

the expression for enttity code is:  (DT_WSRT,100)(TRIM(inst_c)+TRIM(dept_c))

the two columns inst_c and dept_c are from the table in my old db source

anyway, i get a message: column "entity_code" cannot convert between unicode and non-unicode string data types.

i tried using import and export wizard but it didnt help. oce i added the derived column i got that message at the destination.

i tried entity_code in DT_STR cast type but that didn't help either.

TIA