Surprising results from INTEGER_GET_REGION with ALTREP object

Dear Listmembers,

wanting to learn more about ALTREP I wrote the following function to
extract a subsequence from an integer vector:

#include <Rinternals.h>

SEXP integer_get_region(SEXP _x, SEXP _i, SEXP _n) {
   int i = INTEGER(_i)[0];
   int n = INTEGER(_n)[0];
   SEXP result = PROTECT(Rf_allocVector(INTSXP, n));
   INTEGER_GET_REGION(_x, i, n, INTEGER(result));
   UNPROTECT(1);
   return result;
}

For "shorter" vectors, the result is as expected:

dyn.load("altrep_int_region.so")
.Call("integer_get_region", 1:1e9, 0L, 10L)

  [1]  1  2  3  4  5  6  7  8  9 10

But not for "longer" vectors:

.Call("integer_get_region", 1:1e10, 0L, 10L)

  [1]          0 1072693248          0 1073741824          0 1074266112
  [7]          0 1074790400          0 1075052544


Am I doing something wrong or is this a bug? I am using

R.version.string

[1] "R version 3.5.2 (2018-12-20)"

Thanks
Ralf