Sorry, I didn't finish the post. I found that eventually the sliderChangeListener makes this call sliding = true; setValue(newValue("" + d)); sliding = false; On my system since d was a double it creates a decimal string. Eventually this goes to Properties.setValue() where it tries to parse the string as as an integer. When it does this I get a NumberFormatException. Incidentally, I get the same exception when I type a decimal (or any non integer) into the text field for the property inspector. I fixed the slider problem by casting the "d" variable to integer before the string concatenation if the domain isn't a double. I haven't yet fixed the exception when the number gets typed in. Is this a bug or am I doing something wrong?