bpftrace aborts if positional parameter used and no parameter supplied

[tyroguru]

# cat b.t
tracepoint:syscalls:sys_enter_*
/pid == $1/
{
  @[pid,tid,comm] = count();
}

#  /tmp/bpftrace ./b.bt
terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 0) >= this->size() (which is 0)
Aborted (core dumped)