How to change datatype of column in SQL Server using ALTER command

HI

i have image column in my table that is nvarchar(50)

now i want change type i wrote this code

use behtop 
go

alter table House_p
alter column image varbinary(MAX)

but this error occur 

Implicit conversion from data type nvarchar to varbinary(max) is not allowed. Use the CONVERT function to run this query.

what can i do?

Best regards