panic: Incrementing non-positive ref count

goroutine 457931 [running]:
gvisor.googlesource.com/gvisor/pkg/refs.(*AtomicRefCount).IncRef(0xc420d31680)
	pkg/refs/refcounter.go:211 +0x5a
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).Fork(0xc4251bf900, 0xc817c0, 0xc420fddb00, 0x0, 0x0, 0x0)
	pkg/sentry/mm/lifecycle.go:90 +0x471
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*TaskContext).Fork(0xc420fde230, 0xc817c0, 0xc420fddb00, 0xc4251bff00, 0xc421287910, 0x713675, 0xc4251bf900)
	pkg/sentry/kernel/task_context.go:89 +0x16d
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).Clone(0xc420fddb00, 0xc421287ac8, 0x0, 0x0, 0x0, 0x0)
	pkg/sentry/kernel/task_clone.go:203 +0x1b1
gvisor.googlesource.com/gvisor/pkg/sentry/syscalls/linux.clone(0xc420fddb00, 0x800000, 0x20000000, 0x20000040, 0x20000080, 0x200000c0, 0x40000002, 0x0, 0xd094, 0x0)
	pkg/sentry/syscalls/linux/sys_thread.go:157 +0x21b
gvisor.googlesource.com/gvisor/pkg/sentry/syscalls/linux.Clone(0xc420fddb00, 0x800000, 0x20000000, 0x20000040, 0x20000080, 0x200000c0, 0x0, 0x428854, 0xbe99d0, 0xc421287c20, ...)
	pkg/sentry/syscalls/linux/sys_thread.go:171 +0x67
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).executeSyscall(0xc420fddb00, 0x38, 0x800000, 0x20000000, 0x20000040, 0x20000080, 0x200000c0, 0x0, 0xc42028c7e0, 0xbc09c0, ...)
	pkg/sentry/kernel/task_syscall.go:162 +0x307
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).doSyscallInvoke(0xc420fddb00, 0x38, 0x800000, 0x20000000, 0x20000040, 0x20000080, 0x200000c0, 0x0, 0x200000c0, 0x0)
	pkg/sentry/kernel/task_syscall.go:278 +0x62
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).doSyscallEnter(0xc420fddb00, 0x38, 0x800000, 0x20000000, 0x20000040, 0x20000080, 0x200000c0, 0x0, 0xc748a0, 0xc421287e18)
	pkg/sentry/kernel/task_syscall.go:241 +0x91
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).doSyscall(0xc420fddb00, 0x2, 0xc4206987e0)
	pkg/sentry/kernel/task_syscall.go:216 +0x10c
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*runApp).execute(0x0, 0xc420fddb00, 0xc748a0, 0x0)
	pkg/sentry/kernel/task_run.go:217 +0xe56
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).run(0xc420fddb00, 0x36ff)
	pkg/sentry/kernel/task_run.go:95 +0x174
created by gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).Start
	pkg/sentry/kernel/task_start.go:258 +0x100