TypeError: Cannot call method 'collection' of undefined

[tau0]

Randomly occurs:

/var/www/prerender/node_modules/prerender-mongodb-cache/lib/mongoCache.js:46                                                                                                                           │······························································
        database.collection('pages', function(err, collection) {                                                                                                                                       │······························································
                 ^                                                                                                                                                                                     │······························································
TypeError: Cannot call method 'collection' of undefined                                                                                                                                                │······························································
    at Object.mongo_cache.get (/var/www/prerender/node_modules/prerender-mongodb-cache/lib/mongoCache.js:46:18)                                                                                        │······························································
    at Object.module.exports.beforePhantomRequest (/var/www/prerender/node_modules/prerender-mongodb-cache/lib/mongoCache.js:28:20)                                                                    │······························································
    at next (/var/www/prerender/lib/server.js:62:20)                                                                                                                                                   │······························································
    at next (/var/www/prerender/lib/server.js:64:13)                                                                                                                                                   │······························································
    at next (/var/www/prerender/lib/server.js:64:13)                                                                                                                                                   │······························································
    at Object.module.exports.beforePhantomRequest (/var/www/prerender/lib/plugins/blacklist.js:13:13)                                                                                                  │······························································
    at next (/var/www/prerender/lib/server.js:62:20)                                                                                                                                                   │······························································
    at Object.server._pluginEvent (/var/www/prerender/lib/server.js:69:5)                                                                                                                              │······························································
    at Object.server.onRequest (/var/www/prerender/lib/server.js:129:10)                                                                                                                               │······························································
    at Server.wrapper (/var/www/prerender/node_modules/lodash/index.js:3057:19)