I'm seeing a crash/panic when I'm running on kevin with a 12GB qcow disk

thread 'virtio_blk' panicked at 'attempt to subtract with overflow', qcow/src/qcow.rs:305:20
note: Run with `RUST_BACKTRACE=1` for a backtrace.


and here's the backtrace
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:380
   3: std::panicking::default_hook
             at libstd/panicking.rs:396
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:576
   5: std::panicking::begin_panic
             at libstd/panicking.rs:537
   6: std::panicking::begin_panic_fmt
             at libstd/panicking.rs:521
   7: rust_begin_unwind
             at libstd/panicking.rs:497
   8: core::panicking::panic_fmt
             at libcore/panicking.rs:71
   9: core::panicking::panic
             at libcore/panicking.rs:51
  10: qcow::QcowFile::limit_range_file
             at qcow/src/qcow.rs:305
  11: <qcow::QcowFile as std::io::Write>::write
             at qcow/src/qcow.rs:511
  12: std::io::Write::write_all
             at /var/tmp/portage/dev-lang/rust-1.25.0/work/rustc-1.25.0-src/src/libstd/io/mod.rs:1099
  13: sys_util::mmap::MemoryMapping::write_from_memory
             at /mnt/host/source/src/platform/crosvm/sys_util/src/mmap.rs:314
  14: sys_util::guest_memory::GuestMemory::write_from_memory::{{closure}}
             at /mnt/host/source/src/platform/crosvm/sys_util/src/guest_memory.rs:347
  15: sys_util::guest_memory::GuestMemory::do_in_region
             at /mnt/host/source/src/platform/crosvm/sys_util/src/guest_memory.rs:386
  16: sys_util::guest_memory::GuestMemory::write_from_memory
             at /mnt/host/source/src/platform/crosvm/sys_util/src/guest_memory.rs:346
  17: devices::virtio::block::Request::execute
             at /mnt/host/source/src/platform/crosvm/devices/src/virtio/block.rs:168
  18: <devices::virtio::block::Worker<T>>::process_queue
             at /mnt/host/source/src/platform/crosvm/devices/src/virtio/block.rs:196
  19: <devices::virtio::block::Worker<T>>::run
             at /mnt/host/source/src/platform/crosvm/devices/src/virtio/block.rs:269
  20: <devices::virtio::block::Block<T> as devices::virtio::mmio::VirtioDevice>::activate::{{closure}}
             at /mnt/host/source/src/platform/crosvm/devices/src/virtio/block.rs:391