[R] How do I turn NA's to zeroes when a combination lacks one element?

[C_Crown]

Hi all, I am  very new to R. I am taking a course and am trying to complete
my first assignment. 
For the assignment I have to get the program to generate different color
combinations possible for a sample size of 55 with the probabilities of each
color being chosen as: .24, .24, .16, .20, .13, .14. Here is what I've come
up with... 
 
sample.size<- 55
MM.probability<- c(.24, .24, .16, .20, .13, .14)
MM.color<- c('Bl','Br','G','O','R','Y')
mmtable<- matrix(nrow = 1000, ncol = 6)
for(i in 1:1000){
combinations<- sample(MM.color, sample.size, replace = T, prob =
MM.probability)
mmtable[i,]<-table(combinations)
colnames(mmtable)<- c("Bl","Br","G","O","R","Y")
}

I feel like it should work, but every time I run it, it usually only goes so
far (maybe to row 350, or 450, sometimes it completes with no problem)
before I start getting "NA" in every column of every row. I also get this
error message "Error in mmtable[i, ] <- table(combinations) :  number of
items to replace is not a multiple of replacement length"
Someone suggested that it is because the program is coming upon a
combination that is missing one of the colors, so I'd have to instruct it to
put a zero in place of the missing color so the simulation can continue,
which completely makes sense. But I've been trying and can't figure out how
to do it. I tried "mmtable[is.na(mmtable)]<-0", but then I just get zeroes
everywhere instead of NA's, and "mmtable[na.omit(mmtable)]" gives me the
same as no instruction at all. Do you know what the right notation would be?
I also need to use the script to determine the probability of getting the
combination with proportions: .182, .164, .309, .145, .091, .1090. I've also
tried a few things for this and am coming up with nothing. Sorry if this is
a really simple question, I am sure most of you could do this in your sleep,
but it's all very new to me. Thanks in advance. 

-C

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