After running for a few minutes doing some DB operations, I attached the debugger and found out there are NREs happening in the background at the following stack trace:
MongoDB.Driver.DLL!MongoDB.Driver.Internal.MongoConnection.Open() Line 362 + 0x5 bytes C#
MongoDB.Driver.DLL!MongoDB.Driver.Internal.MongoConnection.GetNetworkStream() Line 522 C#
MongoDB.Driver.DLL!MongoDB.Driver.Internal.MongoConnection.SendMessage(MongoDB.Driver.Internal.MongoRequestMessage message, MongoDB.Driver.SafeMode safeMode) Line 473 + 0xc bytes C#
MongoDB.Driver.DLL!MongoDB.Driver.Internal.MongoConnection.RunCommand(string collectionName, MongoDB.Driver.QueryFlags queryFlags, MongoDB.Driver.CommandDocument command, bool throwOnError) Line 389 + 0x3f bytes C#
MongoDB.Driver.DLL!MongoDB.Driver.MongoServerInstance.Ping() Line 217 + 0x2c bytes C#
MongoDB.Driver.DLL!MongoDB.Driver.MongoServerInstance.VerifyState() Line 236 + 0xc bytes C#
MongoDB.Driver.DLL!MongoDB.Driver.Internal.MongoConnectionPool.TimerCallback(object state) Line 344 C#
mscorlib.dll!System.Threading.ExecutionContext.runTryCode(object userData) + 0x178 bytes

It appears that _serverInstance.EndPoint is null for some reason. I'm using git version 77ac96b73b3fe9f4517cc79d3c72c2d87e1bbc9a.