IllegalArgumentException when trying to use Forester

I’m trying to use Forester to generate grass and my program runs, but during forester.update(tpf) it throws the following exception:

SEVERE: null
java.lang.IllegalArgumentException
	at java.nio.Buffer.position(Buffer.java:236)
	at forester.image.formatreader.RDR_BGR8.getColor(RDR_BGR8.java:57)
	at forester.image.ImageReader.getColor(ImageReader.java:110)
	at forester.image.DensityMap.getDensityUnfiltered(DensityMap.java:83)
	at forester.grass.algorithms.GPAUniform.generateGrassData(GPAUniform.java:68)
	at forester.grass.GrassGeometryGenerator.createGrassGeometry(GrassGeometryGenerator.java:91)
	at forester.grass.GrassLoader$LoadTask.call(GrassLoader.java:235)
	at forester.grass.GrassLoader$LoadTask.call(GrassLoader.java:192)
	at java.util.concurrent.FutureTask.run(FutureTask.java:262)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:744)

Was wondering if anyone had encountered this before and knew what the problem is. Thanks.