[R] numeric(0)

[dax42]

Dear all,

I am trying to calculate a score for a string sequence consisting of 
the following four letters: ACGT.
I have got a matrix giving the scores for each pair of letters.
So for example the string ACCT has got the pairs: AC, CC and CT.

The matrix has got the following form:
names<-c("A","C","G","T");
mscore<-matrix(0,4,4);
rownames(mscore)<-names;
colnames(mscore)<-names;

So for the first example pair above I could get the score contained in 
the matrix with the following code:
 >> mscore["A","C"]

I am now trying to sum up all the scores with the following code:

score<-0;

for(j in 1:length(sequence)-1){
	score<-score+mscore[sequence[j],sequence[j+1]];
}

where sequence is the string sequence.
Unfortunately, it does not work and I get the following error message:

Error in "[<-"(`*tmp*`, 1, i, value = numeric(0)) :
	nothing to replace with

What does this mean? Strangely, the command
"print(score+mscore[sequence[j],sequence[j+1]])"
works, so it really is the assignment which won't work. Why is that?
I am running R 2.0 series (GUI version) on Mac OSX 10.3.7.

Any suggestions are welcome.
Thanks a lot,
Dax