[Java-ML-support] NullPointerException during NaiveBayesClassifier.buildClassifier()

I am new to Java-ML, so I apologize if the answer to this is trivial:

I'm trying to construct and use a Classifier in the following way:

        // Construct an instance of the selected classifier
        Classifier c = constructClassifier(classifier); // This returns a
NaiveBayesClassifier
        ...
        // Build a training Dataset for the symbol using the historical data
        Dataset trainingData = buildTrainingDataset(dataArr,
sampleIntervalWidth, futureDays);
        ...
        // Train the classifier
        c.buildClassifier(trainingData); // NullPointerException happens
here
        ...
        // Now, classify against the latest sample
        DenseInstance latestSample = new
DenseInstance(buildSampleDoubleArr(dataArr, dataArr.length -
sampleIntervalWidth - 1, sampleIntervalWidth));
        ...
        // Store the result
        results.put(s, (Double)c.classify(latestSample));

Each time I try this, I seem to be getting a NullPointerException from the
innards of AbstractBayesianClassifier with the following stack trace:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at
net.sf.javaml.classification.bayes.AbstractBayesianClassifier.calculateClassFreqs(AbstractBayesianClassifier.java:207)
at
net.sf.javaml.classification.bayes.AbstractBayesianClassifier.buildClassifier(AbstractBayesianClassifier.java:103)
at
net.sf.javaml.classification.bayes.NaiveBayesClassifier.buildClassifier(NaiveBayesClassifier.java:42)
at tdeering.iastate.cs572.ainvestor.engine.Engine.rank(Engine.java:106)

Can anyone tell me what I am doing wrong? Am I missing a step where I tell
my Classfier about all possible classes?

Thanks,

Tom