Smooth.ppp problem

Dear all,

I am using the package spatstat and trying to smooth a pointpattern. When I try to smooth my data.ppp I keep receiving this message:

Error in Ops.data.frame(marx, yhat) : 
  - only defined for equally-sized data frames

I don?t really get it? I don?t have any ?rejects?.

shape.owin<-as(shape.shp,?owin?)
pts <- coordinates(shape.shp)
p <- ppp(pts[,1],pts[,2],window=spain.owin,marks=DF)

any ideas?

Ale