The null values are treated as deletes when they are written to the store.
You can see here:

On Tue, 8 Aug 2017 at 11:22 Bart Vercammen <b...@cloutrix.com> wrote:

> Hi,
> I noticed the following:
> When a kafka streams application starts, it will restore its state in its
> state-stores (from the log-compacted kafka topic).  All good so far, but I
> noticed that the 'deleted' entries are actually read in into the store as
> 'key' with value:`null`
> Is this expected behaviour?  I would assume that 'null' values are ignored
> when restoring the state as this is exactly how the entries are deleted on
> the log-compacted kafka-topic.
> When the compaction has run on the kafka topic, all is fine, but when the
> segment is not compacted yet, these null values are read in.
> Greets,
> Bart

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