[R] Calculate correctly, but gives an error message

[TEMPL Matthias]

Hi!

I will calculate sum´s in the following way:
E.g.: 


a <- rpois(100,20)
b <- rpois(100,5)

x <- data.frame(cbind(a,b))

# the sum´s should be calculated based on a.

attach(x) 

sort.nace <- unique(sort(x[,1]))

sum1 <- matrix(ncol=1, nrow=length(sort.nace))

# I calculate the sum of all values of b, which have the same category in a. Eg.:

sum1[1,] <- sum(subset(x, a==sort.nace[1], select=b), na.rm=TRUE)

# or:

sum1[5,] <- sum(subset(x, a==sort.nace[5], select=b), na.rm=TRUE)

# all is ok.
# but when I do it in a loop, I get an error message.

sum1 <- matrix(ncol=1, nrow=length(sort.nace))
for(i in 1:length(nace)){
	sum1[i,] <- sum(subset(x, a==sort.nace[i], select=b), na.rm=TRUE)
	}
#Error in Summary.data.frame(..., na.rm = na.rm) : 
#        only defined on a data frame with all numeric or complex variables

# but sum1 was correct calculated(!):

# sum1
#       [,1]
#  [1,]    3
#  [2,]    4
#  [3,]    8
#  [4,]    7
#  [5,]   41
#  [6,]   57
#  [7,]   38
#  [8,]   15
#  [9,]   44
# [10,]   72
# [11,]   57
# [12,]   27
# [13,]   12
# [14,]   24
# [15,]   29
# [16,]   16
# [17,]   19
# [18,]    3
# [19,]    4
# [20,]    8
# [21,]    6

The variables of x are of class numeric. 
My function, which should calculate these sum´s, does not work, because of this error.

Can anybody please tell me, what I´m doing wrong?

(I think it would be better to write a sapply instead of the loop, but I have two functions (sum and subset) and it is very hard for me to use the sapply here correctly)

Thanks,
Matthias