Fixed the exception when saving a new session
Fixed the exception when saving a new session

Signed-off-by: Nils Hasenbanck <ad3fa8d847df2b57853a376ad688e4be8041ecd4@hasenbanck.de>

noirbizarre/mongoengine,jimmyshen/mongoengine,iandd0824/mongoengine,depop/mongoengine-stripped,MRigal/mongoengine,gavincabbage/mongoengine,eli-b/mongoengine,yarneo/mongoengine,ProgressivePlanning/mongoengine,blaggacao/mongoengine,ms5/mongoengine,muhammad-ammar/mongoengine,MakerReduxCorp/mongoengine,9nix00/mongoengine,starsirius/mongoengine,Davidrjx/mongoengine,MongoEngine/mongoengine,MakerReduxCorp/mongoengine,closeio/mongoengine,cogniteev/mongoengine,Catstyle/mongoengine,larsbutler/mongoengine,gmacon/mongoengine,touilleMan/mongoengine,RussellLuo/mongoengine,DavidBord/mongoengine,Multiposting/mongoengine,nathanial/mongoengine,sourcelair/mongoengine,losintikfos/mongoengine,nicolascine/mongoengine,khallaghi/mongoengine,prijindal/mongoengine,ConsumerPhysics/mongoengine,hmarr/mongoengine,elephanter/mongoengine,Davidrjx/mongoengine,reallistic/mongoengine,WingGao/mongoengine,cogniteev/mongoengine