[R] Rgraphviz and NA indices error

[Sun Shine]

Hi list

Can someone help me debug the following please:

Having downloaded and installed the bioconductor packages and Rgraphviz, 
I am attempting to plot a network graph showing the relation among 
chosen words in the corpus of text data.

I first did this:
 > plot(dtm, terms=findFreqTerms(dtm, lowfreq=100) [1:30], 
corThreshold=0.75)

and received the error message:

Error in `[.simple_triplet_matrix`(m, , terms) : NA indices not allowed.


My next step was to remove any NA indices (although to be honest, this 
is more of a stab in the dark because there shouldn't be any NA values 
in the corpus):

 > docsNA <- (docs[!is.na(docs)])

Then redid the DTM with the NA values removed
 > dtmNA <- DocumentTermMatrix(docsNA)

Then re-ran Rgraphviv with the new set
 > plot(dtmNA, terms=findFreqTerms(dtmNA, lowfreq=100) [1:10], 
corThreshold=0.5)

But, still get an error:
Error in `[.simple_triplet_matrix`(m, , terms) : NA indices not allowed.

I have not been successful in finding out why this error persists nor 
what to do about it.

Anyone have any ideas to progress past this issue?

Thanks

Sun