Fatal error: unexpected end of JSON input

I'm using current top of the tree of couchbase-net-client (e205bc140519d117cbacb93ef563e1fe4248545b)

With following 2 node cluster

node 1 server version 4.5 GA : kv

node 2 server version 4.5 GA : kv,index,n1ql

and extra 2 node cluster with 5.0.0-3217 to be swap rebalanced.

Test was started with 2 node cluster then follow this

1. swap rebalance kv node by removing 4.5 and adding 5.0
2. add 5.0 and rebalance then copy index with different index name ( at this point, we have two index with same index value)
3. remove 4.5GA and rebalance (at this point, we have completed upgrade to 5.0 so we have just 2 node cluster of 5.0)

Expected: no errors

Actual: Fatal error continues even after upgrade was completed. Success also continues but total success is smaller than before upgrade.

client log is attached