Error 500 on "Not enough coins to buy a plot."

[Bersam]

Reproduce Bug:
on this page, https://openfarmgame.com/buy-plot
when you don't have enough coin to buy a new plot, it raise Internal Server Error, 500!

Firebug:
"NetworkError: 500 Internal Server Error - https://openfarmgame.com/buy-plot"

Output HTML:
Express
500 Error: Not enough coins to buy a plot.

at exports.handleBuyPlot (/home/ubuntu/openfarmgame/routes/index.js:399:14)
at callbacks (/home/ubuntu/openfarmgame/node_modules/express/lib/router/index.js:272:11)
at async.waterfall.userRequired (/home/ubuntu/openfarmgame/app.js:222:17)
at callbacks (/home/ubuntu/openfarmgame/node_modules/express/lib/router/index.js:272:11)
at async.waterfall.userAuth (/home/ubuntu/openfarmgame/app.js:208:25)
at DatabankObject.subClass.Cls.get (/home/ubuntu/openfarmgame/node_modules/databank/lib/databankobject.js:110:33)
at passThru0 (/home/ubuntu/openfarmgame/node_modules/databank/lib/databankobject.js:44:5)
at DatabankObject.subClass.Cls.get (/home/ubuntu/openfarmgame/node_modules/databank/lib/databankobject.js:106:27)
at MongoDatabank.read (/home/ubuntu/openfarmgame/node_modules/databank/node_modules/databank-mongodb/lib/mongodb.js:282:17)
at Collection.findOne (/home/ubuntu/openfarmgame/node_modules/databank/node_modules/databank-mongodb/node_modules/mongodb/lib/mongodb/collection.js:1010:5)