IllegalArgumentException in ChartMouseEvent

In ChartPanel.mouseMoved there's a line of code which
create a new ChartMouseEvent using as first parameter
the object returned by getChart(). For getChart() is
legal to returns null if the chart is null, but
ChartMouseEvent's constructor calls the parent
constructor which throws an IllegalArgumentException if
the object passed in is null.
Here's the stack trace:

java.lang.IllegalArgumentException: null source
at java.util.EventObject.<init>(EventObject.java:38)
at
org.jfree.chart.ChartMouseEvent.<init>(ChartMouseEvent.java:83)
at
org.jfree.chart.ChartPanel.mouseMoved(ChartPanel.java:1692)
at
java.awt.AWTEventMulticaster.mouseMoved(AWTEventMulticaster.java:271)
at
java.awt.Component.processMouseMotionEvent(Component.java:5533)
at
javax.swing.JComponent.processMouseMotionEvent(JComponent.java:3144)
at java.awt.Component.processEvent(Component.java:5257)
at java.awt.Container.processEvent(Container.java:1966)
at
java.awt.Component.dispatchEventImpl(Component.java:3955)
at
java.awt.Container.dispatchEventImpl(Container.java:2024)
at java.awt.Component.dispatchEvent(Component.java:3803)
at
java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4212)
at
java.awt.LightweightDispatcher.processMouseEvent(Container.java:3905)
at
java.awt.LightweightDispatcher.dispatchEvent(Container.java:3822)
at
java.awt.Container.dispatchEventImpl(Container.java:2010)
at java.awt.Window.dispatchEventImpl(Window.java:1774)
at java.awt.Component.dispatchEvent(Component.java:3803)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:463)
at
<deleted entry>
at
java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:242)
at
java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:163)
at
java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:157)
at
java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:149)
at
java.awt.EventDispatchThread.run(EventDispatchThread.java:110)