Serializer is invoked with null when foreignValue=null

I'm using the Ktable's foreign key join (ref: KIP-213).  When I try to leftJoin two KTables, and the left side's foreignKey does not have a key match in the right (other) KTable, the Join operation invokes the serializer with a foreignValue=null.  This then causes a NullPointerException in the provided (my) serializer.

Stack Trace is attached.  Notes:

In method RecordCollectorImpl#send() (around line 97), inspecting 'value' shows:

SubscriptionResponseWrapper{version=0, foreignValue=null, originalValueHash=[-5192323959200117799, -2503872568163857643]} 

This foreignValue is passed to Serializer.java#serialize() as 'data'.  Ultimately, it ends up in SubscriptionResponseWrapperSerde#serialize (#72) where 'serliazedData` is null.  This is what is passed to my serializer.

This exception is readily reproducible; just set up the left message-value to include a foreign key value that does not exist on the right side (the 'other' table).