Problem with raster calculation

Dear all

I am experiencing a strange problem when calculating in the raster package.
Let me explain it with a simple example;

a <- c(1,1,1,0,0,0,2,2,2)
a <- matrix(a,3,3,byrow=T)
a <- raster(a)
a []

[1] 1 1 1 0 0 0 2 2 2

b <- 10 - a
b []

[1] ?-9 ?-9 ?-9 -10 -10 -10 ?-8 ?-8 ?-8

As you can see, the obtained values are not correct. I cannot see why
this should be.
Can anybody help?

Aidin Niamir