Exception while invoking method 'serversession_set' Error: Did not check() all arguments during call to 'serversession_set'

Getting the following error

I20150905-22:30:32.082(3)? Exception while invoking method 'serversession_set' Error: Did not check() all arguments during call to 'serversession_set'
I20150905-22:30:32.084(3)? at [object Object]._.extend.throwUnlessAllArgumentsHaveBeenChecked (packages/check/match.js:357:1)
I20150905-22:30:32.084(3)? at Object.Match._failIfArgumentsAreNotAllChecked (packages/check/match.js:112:1)
I20150905-22:30:32.084(3)? at maybeAuditArgumentChecks (packages/ddp/livedata_server.js:1614:1)
I20150905-22:30:32.084(3)? at packages/ddp/livedata_server.js:648:1
I20150905-22:30:32.084(3)? at [object Object]._.extend.withValue (packages/meteor/dynamics_nodejs.js:56:1)
I20150905-22:30:32.084(3)? at packages/ddp/livedata_server.js:647:1
I20150905-22:30:32.084(3)? at [object Object]._.extend.withValue (packages/meteor/dynamics_nodejs.js:56:1)
I20150905-22:30:32.084(3)? at [object Object]._.extend.protocol_handlers.method (packages/ddp/livedata_server.js:646:1)
I20150905-22:30:32.084(3)? at packages/ddp/livedata_server.js:546:1

Start by reproducing the serversession_set method call and read the stack trace entries in packages/check/match.js and packages/ddp/livedata_server.js. Identify which argument is not checked and confirm that the method completes without the Did not check() all arguments error.

Written by the indexing model from the issue text.