tar-tree.c dereferencing pointer to incomplete type

Hello:

I'm building v 1.8.1.1 on Android. The build chokes on the builtins phase on tar-tree.c with:

    CC builtin/tar-tree.o
builtin/tar-tree.c: In function 'cmd_get_tar_commit_id':
builtin/tar-tree.c:93:12: error: dereferencing pointer to incomplete type
make: *** [builtin/tar-tree.o] Error 1

The line that's causing this in tar-tree.c is:

if (header->typeflag[0] != 'g')

from

int cmd_get_tar_commit_id(int argc, const char **argv, const char *prefix)
{
	char buffer[HEADERSIZE];
	struct ustar_header *header = (struct ustar_header *)buffer;
	char *content = buffer + RECORDSIZE;
	ssize_t n;

	if (argc != 1)
		usage(builtin_get_tar_commit_id_usage);

	n = read_in_full(0, buffer, HEADERSIZE);
	if (n < HEADERSIZE)
		die("git get-tar-commit-id: read error");
	if (header->typeflag[0] != 'g')
		return 1;
	if (memcmp(content, "52 comment=", 11))
		return 1;

	n = write_in_full(1, content + 11, 41);
	if (n < 41)
		die_errno("git get-tar-commit-id: write error");

	return 0;
}

But why?

What's messed up with my setup.

I can compile no probs on other machines. I'm trying to get my head round this, but don't see the problem?