R help - May 2004 - using trace() on S4 coerce method

I'm trying to use trace() on an S4 coerce method, but get the error

    Error in bindingIsLocked(what, whereM) : no binding for "coerce"

What am I doing wrong? Example code follows.

(I've googled the R mailing lists for "trace coerce" and
"trace
bindingisLocked" without finding anything relevant. Perhaps I should 
re-define the method in terms of an ordinary function and trace the later 
function, as suggested by John Chambers to another user:
https://stat.ethz.ch/pipermail/r-devel/2003-August/027249.html
But it would be nice to be able to trace the method directly.)

Thanks
Gordon

 > setClass("MyClass1",representation(a="numeric"))
[1] "MyClass1"
 > setClass("MyClass2",representation(a="character"))
[1] "MyClass2"
 > setAs("MyClass1","MyClass2",function(from,to) 
new(to,a=as.character(from at a)))
 > x <- new("MyClass1",a=20)
 > as(x,"MyClass2")
An object of class "MyClass2"
Slot "a":
[1] "20"

 > tracingState(on=TRUE)
[1] FALSE
 > getMethod("coerce",c("MyClass1","MyClass2"))
Method Definition (Class "MethodDefinition"):

function (from, to = "MyClass2", strict = TRUE)
new(to, a = as.character(from at a))

Signatures:
         from       to
target  "MyClass1" "MyClass2"
defined "MyClass1" "MyClass2"
 >
trace("coerce",browser,signature=c("MyClass1","MyClass2"))
Error in bindingIsLocked(what, whereM) : no binding for "coerce"

 > version
          _
platform i386-pc-mingw32
arch     i386
os       mingw32
system   i386, mingw32
status
major    1
minor    9.0
year     2004
month    04
day      12
language R