Panic while flushing chunks

[pstibrany]

This panic happened while calling /flush endpoint on ingester in internal cluster. Ingester was running grafana/cortex-cortex:r92-a0674c0c2 (https://github.com/grafana/cortex/commits/a0674c0c2f528f8aa5297768604e17ca7ebb047f)

level=info ts=2020-06-18T08:23:49.848675718Z caller=flush.go:42 msg="starting to flush all the chunks"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x279a9b7]

goroutine 1288 [running]:
github.com/cortexproject/cortex/pkg/ingester.(*Ingester).flushChunks(0xc0006e8800, 0x3ee0260, 0xc002814d20, 0xc000e0c6b0, 0x3, 0x87ff6a90a1aa9836, 0xc0191ae160, 0xb, 0xb, 0xc1c44a5180, ...)
	/go/src/github.com/cortexproject/cortex/pkg/ingester/flush.go:344 +0x637
github.com/cortexproject/cortex/pkg/ingester.(*Ingester).flushUserSeries(0xc0006e8800, 0x20, 0xc000e0c6b0, 0x3, 0x87ff6a90a1aa9836, 0x1, 0x0, 0x0)
	/go/src/github.com/cortexproject/cortex/pkg/ingester/flush.go:291 +0x843
github.com/cortexproject/cortex/pkg/ingester.(*Ingester).flushLoop(0xc0006e8800, 0x20)
	/go/src/github.com/cortexproject/cortex/pkg/ingester/flush.go:205 +0x10a
created by github.com/cortexproject/cortex/pkg/ingester.(*Ingester).startFlushLoops
	/go/src/github.com/cortexproject/cortex/pkg/ingester/ingester.go:256 +0x68