The specifications of methods [1] and [2] are self-contradictory in stating both that "If param is null, a default write param will be used." and "IllegalArgumentException - if any of the following are true: [...] param is null."

[1] http://download.java.net/jdk9/docs/api/javax/imageio/ImageWriter.html#replacePixels-java.awt.image.Raster-javax.imageio.ImageWriteParam-
[2] http://download.java.net/jdk9/docs/api/javax/imageio/ImageWriter.html#replacePixels-java.awt.image.RenderedImage-javax.imageio.ImageWriteParam-