Java tree map trouble

This is a project for a class, and I am totally stuck! I've created a tree map using an input text file. I split the lines into fields so that I could work with 2 elements from the file. It appears to display correctly, but I'm getting an "ArrayIndexOutOfBoundsException: 3" error, and I'm not getting anything returned to my Set. I just can't seem to wrap my brain around this one...can someone take a look at my code and point me in the right direction please?

  [code]
  public static Set<String> agentValue(String inputFileName)
    throws FileNotFoundException
 {
     Set<String> tail = new TreeSet<String>();
     SortedMap<String, Number> agentValues = new TreeMap<String, Number>();
     Scanner in = new Scanner(new File(inputFileName));
 String line = inputFileName;
 String[] fields;

   while (in.hasNextLine())
   {
     line = in.nextLine();
 fields = line.split("\\s");
     String agentId = (fields [3]); 
 Double pValue = Double.parseDouble(fields [2]);

     if (agentValues.containsKey(agentId))
        {
    pValue += agentValues.get(agentId).intValue();
        }
     agentValues.put(agentId, pValue);


 // Create keyMap with all keys and values
        Set<String> keySet = agentValues.keySet();
        for (String key : keySet)
        {
        Number value = agentValues.get(key);
        System.out.println(key + " : " + value);

        tail.add(key + " : " + value);
        }

   }
   return tail;
   }
  [/code]