If the batchSize associated with the initial find command is zero, then the AsyncResultsMerger will send a batchSize of zero to the shards for subsequent getMore commands. Setting a batchSize of zero is illegal for the getMore command and the mongod will return a BadValue error: "Batch size for getMore must be positive, but received: 0".

Instead, the getMore commands forwarded to the shards should not have a batchSize. In order to fix, we should not set the value of ClusterClientCursorParams::batchSize to boost::none rather than zero here:

https://github.com/mongodb/mongo/blob/4a6ff97bd91c340eea7142d13153a495ab3964a3/src/mongo/s/query/cluster_find.cpp#L149