It's possible that the virtual address that you're examining
is not backed by physical memory (yet).  If you do a "vtop"
on the address that gives you the error, you may see it's
simply not mapped.

For example, I created a little program call "bogus" that
does a malloc of 1000000, prints the beginning address
of the region, and then pauses:

main()
{
        char *buf;

        buf = malloc(1000000);
        printf("buf: %lx\n",
buf);
        pause();
}

When I run it, it prints "buf: 2a95586010".  If I look at
it with crash, the first page of data is instantiated, i.e.
it was already in place before the malloc() call, but if
I go to any page beyond that, it does this:

crash> set 13474
    PID: 13474
COMMAND: "bogus"
   TASK: 100310f57f0  [THREAD_INFO: 10005394000]
    CPU: 1
  STATE: TASK_INTERRUPTIBLE
crash> rd 2a95586010
      2a95586010:  0000000000000000                   
........
crash> rd 2a95587010
rd: invalid user virtual address: 2a95587010  type: "64-bit
UVADDR"
crash> vtop 2a95587010
VIRTUAL     PHYSICAL
2a95587010  (not mapped)

   PGD: 279cb550 => 365fa067
   PMD: 365fa550 => 27ce6067
   PTE: 27ce6c38 => 0

      VMA          
START       END     FLAGS
FILE
  1003c5898d8    2a95584000 2a9567c000 100073

crash>

Dave