R: annotate () gives error in R

I am new to R. I have to use POSTagger in my code. I am using openNLP with R. While trying to execute the following example code (in Sample.R file):

library("NLP")
library("openNLP")
s <- paste(c("Pierre Vinken, 61 years old, will join the board as a ",
"nonexecutive director Nov. 29.\n",
"Mr. Vinken is chairman of Elsevier N.V., ",
 "the Dutch publishing group."),
 collapse = "")
s <- as.String(s)

sent_token_annotator <- Maxent_Sent_Token_Annotator()
a1 <- annotate(s, sent_token_annotator)
s[a1]

      

        
        
        
      

    

And running this code from the R Console (using source ("Sample.R")) I get the following error:

Error in as.data.frame.default(x[[i]], optional = TRUE) : 
cannot coerce class "c("Simple_POS_Tag_Annotator", "Annotator")" to a data.frame

      

        
        
        
      

    

Following is the output of the traceback () command:

14: stop(gettextf("cannot coerce class \"%s\" to a data.frame", deparse(class(x))), 
    domain = NA)
13: as.data.frame.default(x[[i]], optional = TRUE)
12: as.data.frame(x[[i]], optional = TRUE)
11: data.frame(x = function (s, a = Annotation()) 
{
    s <- as.String(s)
    y <- f(s)
    n <- length(y)
    id <- .seq_id(next_id(a$id), n)
    type <- rep.int("sentence", n)
    if (is.Annotation(y)) {
        y$id <- id
        y$type <- type
    }
    else if (is.Span(y)) {
        y <- as.Annotation(y, id = id, type = type)
    }
    else stop("Invalid result from underlying sentence tokenizer.")
    if (length(i <- which(a$type == "paragraph"))) {
        a <- a[i]
        a$features <- lapply(annotations_in_spans(y, a), function(e) list(constituents = e$id))
        y <- c(y, a)
    }
    y
}, check.names = FALSE, stringsAsFactors = FALSE)
10: eval(expr, envir, enclos)
9: eval(as.call(c(expression(data.frame), x, check.names = !optional, 
   stringsAsFactors = stringsAsFactors)))
8: as.data.frame.list(x[[i]], optional = TRUE, stringsAsFactors = stringsAsFactors)
7: as.data.frame(x[[i]], optional = TRUE, stringsAsFactors = stringsAsFactors)
6: data.frame(position)
5: annotate(s, sent_token_annotator) at sample.R#11
4: eval(expr, envir, enclos)
3: eval(ei, envir)
2: withVisible(eval(ei, envir))
1: source("sample.R")

      

        
        
        
      

    

What could be wrong? I am using Rx64 3.1.1 on Windows 7. Any help would be much appreciated. Thanks in advance.