DateAdd in Report Builder an SQL Management Studio (TSQL)

Why this doesn't work ? :

@vs - Integer (Parameter)

@datum - DateTime (Parameter)

DATEADD(d, @vs, @datum) - Result: Error converting decimal to datetime

This works: DATEADD(d, 14, @datum)

This also doesn't work in SQL Management Studio

The whole Where-Clause:

... WHERE (r.Termin BETWEEN @datum AND DATEADD(d, @vs, @datum))