S4 classes, creating in C

 
 I am using C code to create an S4 object based on Douglas Bates's example
in his lecture notes on
<http://www.ci.tuwien.ac.at/Conferences/DSC-2003/Tutorials/RExtensions/slide
s.pdf>
http://www.ci.tuwien.ac.at/Conferences/DSC-2003/Tutorials/RExtensions/slides
.pdf

e.g.

SEXP La_DGE_dc(SEXP A)
        {
        SEXP aa = PROTECT(duplicate(A));
        SEXP adims, pivot, val;
        int m, n, info;
       
        if (!isMatrix(aa) || !isReal(aa)) {
                error("A must be a double precision matrix");
                }

        adims = GET_DIM(aa); m = INTEGER(adims)[0]; n = INTEGER(adims)[1];
        pivot = PROTECT(NEW_INTEGER(m < n ? m : n));

        F77_CALL(dgetrf)(&m, &n, REAL(aa), &m, INTEGER(pivot), &info);
       
        check_Lapack_error(info, "dtrtrf");
       
        val = PROTECT(NEW_OBJECT(MAKE_CLASS("LUdecomposition")));

        SET_SLOT(val, install("a"), aa);
        SET_SLOT(val, install("pivot"), pivot);
       
        UNPROTECT(3);
        return val;
        }

LUdecomposition is an S4 class defined as

setClass("LUdecomposition", representation(a="matrix", pivot = "integer"))

This works in R 1.7.0 and R 1.7.1 but if I initialise any of the slots

setClass("LUdecomposition", representation(a="matrix", pivot = "integer"),
                            prototype=list(pivot = NA)                    )

then I get the following error message in R 1.7.1 but not R 1.7.0

Error in makePrototypeFromClassDef(properties, ClassDef, immediate) :
        In making the prototype for class "LUdecomposition" elements of the
prototype
        failed to match the corresponding slot class: pivot (class " integer
")

Why  can I no longer use the prototype to set the default values?