This line will throw an Exception if the driver is talking to a mongos, but not if the driver is talking to a mongod.

For a mongos, the message that the driver is calling .equals on looks like:

"errmsg" : "{ red_0/cailinmac:28000,cailinmac:28001,cailinmac:28002: \"ns not found\" }"

whereas for a mongod the message looks like:

"errmsg" : "ns not found"

This causes an Exception to be raised in the mongos case but not the mongod case.