dataset parameters in Contains clause

I am trying to use a query parameter in a Contains clause but get the error - "The @param SQL construct or statement is not supported."

The query is:

SELECT DISTINCT SURNAME, FIRNAME
FROM        table
WHERE     CONTAINS(SURNAME, @param)

There is a full-text index on the SURNAME column

The non fulltext equivalent 

SELECT DISTINCT SURNAME, FIRNAME
FROM         table
WHERE     (SURNAME LIKE @param)

works perfectly. I have tried everything I can think of, can anyone help me here

Thanks