Nested functions in T-sql

Hi, I am trying to call a scalar user function
from an inline function, but it is not working.
If I declare a variable and assign the function
to it, it works!, but when I try to call the function directly from another function, I get
an error. The code is the following:

Declare @fecha1 datetime
Declare @fecha2 Datetime

Set @fecha2 = '10/02/2008'
-- if I use the function here, it is fine
Set @fecha1 = dbo.MesUnoStr(@fecha2)
Select * From Prop_MP_Per(@fecha1,@fecha2)

-- but if I use the function inside another
-- (as a parameter) I get a parse error
-- (before actually executing the code)
Select * From Prop_MP_Per(dbo.MesUno(@fecha2),@fecha2)

I appreciate If someone could help me.

Thanks for your help!!