NSCell copyWithZone producing EXC_BAD_ACCESS

FROM : Graham
DATE : Sun Nov 11 22:21:38 2007

My application worked without problems in Tiger but crashes in Leopard 
as follows...

I am using an NSBrowser for which I am supplying a custom cell class 
called SelectionCell (inherits from NSBrowserCell).
I set the prototype in the browser's matrix as follows:

[matrix setPrototype:[SelectionCell alloc]];

and this is where I get the crash, with the following stack trace:

#0    0x9006c9a7 in -[NSCell copyWithZone:]
#1    0x9025da12 in -[NSMatrix setPrototype:]
#2    0x00008074 in -[BrowserDelegate 
browser:createRowsForColumn:inMatrix:] at BrowserDelegate.m:262
#3    0x903fc24c in -[NSBrowser _sendDelegateCreateRowsForColumn:inMatrix:]
#4    0x902724b7 in -[NSBrowser addColumn]
#5    0x90271725 in -[NSBrowser loadColumnZero]
#6    0x000056a0 in -[Controller startup] at Controller.m:417
#7    0x000041f9 in -[Controller getToken] at Controller.m:236
#8    0x000046ed in -[Controller carryOn:] at Controller.m:280
#9    0x90114f94 in -[NSApplication sendAction:to:from:]
#10    0x90114ed4 in -[NSControl sendAction:to:]
#11    0x90114d5a in -[NSCell _sendActionFrom:]
#12    0x901143bb in -[NSCell trackMouse:inRect:ofView:untilMouseUp:]
#13    0x90113c12 in -[NSButtonCell trackMouse:inRect:ofView:untilMouseUp:]
#14    0x901134cc in -[NSControl mouseDown:]
#15    0x90111d9b in -[NSWindow sendEvent:]
#16    0x900dea2c in -[NSApplication sendEvent:]
#17    0x9003c705 in -[NSApplication run]
#18    0x900099ba in NSApplicationMain
#19    0x00002638 in main at main.m:6

The setPrototype immediately causes the copyWithZone message to the 
underlying NSCell and this causes a EXC_BAD_ACCESS signal for some 
reason.

Any ideas as to the problem would be appreciated.