Creating ref class with same name as S4 class results in error

It seems that if one overwrites an S4 class with a Reference Class of
the same name that one gets an error in the situation below. I would
have expected that the Reference Class would replace the S4 class in
such a way that one could now use the Reference Class.

A <- setClass("A", representation(x = "ANY"))
x <- A()
A <- setRefClass("A", fields = "a")
x <- A$new(a = 1)

Error in initialize(value, ...) : invalid names for slots of class "A": a

R.version.string

[1] "R version 2.15.2 Patched (2013-01-17 r61672)"

I got the same result on:

 "R Under development (unstable) (2013-01-25 r61745)"

Both were run under Windows 8 64 bit.

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