Problem with SUM() with varbinary data

Hello everybody,

I have a problem, my information in BD is encrypted. but i need to desencryte and convert to numeric and then get a SUM()

I'm trying to do it, eg

SELECT @TOT = SUM(cast(CAST(isnull(decryptbykey(TOT_TRAB_INCLUIDOS),0) as varchar(15)) as float))   from TABLE_XX

or 

 SELECT @TOT_Trabajadores = SUM(cast(CAST(decryptbykey(TOT_TRAB_INCLUIDOS) as varchar(15)) as float))  from TABLE_XX

This error show me: Operand data type varbinary(max) is invalid for sum operator.

Someone may i help me, please :(

Thks so much