title: TermDocumentMatrix sometimes throwing error
body: <p>I am creating a Word Cloud based on Tweets from various different sports teams. This code executes successfully about 1 in 10 times:</p>
  
  <pre><code>handle &lt;- 'arsenal'
  txt &lt;- searchTwitter(handle,n=1000,lang='en')
  t &lt;- sapply(txt,function(x) x$getText())
  t &lt;- gsub('http.*\\s*|RT|Retweet','',t)
  t &lt;- gsub(handle,'',t)
  t_c &lt;- Corpus(VectorSource(t))
  tdm = TermDocumentMatrix(t_c,control = list(removePunctuation = TRUE,stopwords = stopwords("english"),removeNumbers = TRUE, content_transformer(tolower)))
  m = as.matrix(tdm)
  word_freqs = sort(rowSums(m), decreasing=TRUE) 
  dm = data.frame(word=names(word_freqs), freq=word_freqs)
  wordcloud(dm$word, dm$freq, random.order=FALSE, colors=brewer.pal(8, "Dark2"),rot.per=0.5)
  </code></pre>
  
  <p>The other 9 out of 10 times, it throws the following error:</p>
  
  <pre><code>Error in simple_triplet_matrix(i = i, j = j, v = as.numeric(v), nrow = length(allTerms),  : 
    'i, j, v' different lengths
  In addition: Warning messages:
  1: In mclapply(unname(content(x)), termFreq, control) :
    all scheduled cores encountered errors in user code
  2: In simple_triplet_matrix(i = i, j = j, v = as.numeric(v), nrow = length(allTerms),  :
    NAs introduced by coercion
  </code></pre>
  
  <p>Any ideas guys? I've googled, but so far have come up short! Keep in mind I'm an absolute newbie in R!</p>