diagnosing an applet error

[Michael Sims <jellicle@inch.com>]

So, I have an applet on a web page that I'm trying to run, and it keeps 
throwing this error:

UIDefaults.getUI() failed: no ComponentUI class for: 
javax.swing.JMenuItem[,0,0,0x0,invalid,alignmentX=null,alignmentY=null,border=,flags=0,maximumSize=,minimumSize=,preferredSize=,defaultIcon=,disabledIcon=,disabledSelectedIcon=,margin=null,paintBorder=false,paintFocus=false,pressedIcon=,rolloverEnabled=false,rolloverIcon=,rolloverSelectedIcon=,selectedIcon=,text=]
java.lang.Error
	at javax.swing.UIDefaults.getUIError(Unknown Source)
	at javax.swing.UIDefaults.getUI(Unknown Source)
[...lots more...]
	at java.awt.EventDispatchThread.run(Unknown Source)

I know that this applet works properly when using Sun's JRE 1.4.1_03 
installed on Windows/Internet Explorer.  I assume therefore that the 
applet is coded properly.  I've tried numerous different JRE's with 
Debian: 1.4.0.99, 1.4.1 (several), 1.4.2 from java.sun.com -- they all 
give the same error.  I've also tried to install various Debian packages 
that appeared to be Java class libraries, in case I was missing 
something - nothing seems to help.

Can anyone tell me what's wrong/why this doesn't work under Debian?

-- 
Michael Sims