deeplearning4j: SkipGram NullPointerException

I’m getting the following NullPointerException when calling Word2Vec.fit() after reloading the model.

Exception in thread "VectorCalculationsThread 0" Exception in thread "VectorCalculationsThread 3" java.lang.RuntimeException: java.lang.NullPointerException
	at org.deeplearning4j.models.sequencevectors.SequenceVectors$VectorCalculationsThread.run(SequenceVectors.java:1100)
Caused by: java.lang.NullPointerException
	at org.deeplearning4j.models.embeddings.learning.impl.elements.SkipGram.learnSequence(SkipGram.java:167)
	at org.deeplearning4j.models.sequencevectors.SequenceVectors.trainSequence(SequenceVectors.java:303)
	at org.deeplearning4j.models.sequencevectors.SequenceVectors$VectorCalculationsThread.run(SequenceVectors.java:1075)
java.lang.RuntimeException: java.lang.NullPointerException
	at org.deeplearning4j.models.sequencevectors.SequenceVectors$VectorCalculationsThread.run(SequenceVectors.java:1100)
Caused by: java.lang.NullPointerException
	at org.deeplearning4j.models.embeddings.learning.impl.elements.SkipGram.learnSequence(SkipGram.java:167)
	at org.deeplearning4j.models.sequencevectors.SequenceVectors.trainSequence(SequenceVectors.java:303)
	at org.deeplearning4j.models.sequencevectors.SequenceVectors$VectorCalculationsThread.run(SequenceVectors.java:1075)

The model is loaded using the following command:

Word2Vec word2Vec = WordVectorSerializer.readWord2VecModel(new File(modelPath), true);