When mongo throws duplicate key error, this toError function attempts to write 'toJSON' property.

See stacktrace:

TypeError: Cannot assign to read only property 'toJSON' of MongoError: E11000 duplicate key error index: postujHovna.poo_states.$type_1_owner_1_photo_1 dup key: { : "gone", : ObjectId('55ca6213ae46b3242b6d8d79'), : ObjectId('55ede6f5e1658d5c2c89de67') }
at toError (c:\WebstormProjects\postuj-hovna-server\node_modules\moonridge\node_modules\mongoose\node_modules\mongodb\lib\utils.js:121:14)
at c:\WebstormProjects\postuj-hovna-server\node_modules\moonridge\node_modules\mongoose\node_modules\mongodb\lib\bulk\unordered.js:456:64
at resultHandler (c:\WebstormProjects\postuj-hovna-server\node_modules\moonridge\node_modules\mongoose\node_modules\mongodb\lib\bulk\unordered.js:409:5)
at c:\WebstormProjects\postuj-hovna-server\node_modules\moonridge\node_modules\mongoose\node_modules\mongodb\node_modules\mongodb-core\lib\topologies\server.js:756:13
at Callbacks.emit (c:\WebstormProjects\postuj-hovna-server\node_modules\moonridge\node_modules\mongoose\node_modules\mongodb\node_modules\mongodb-core\lib\topologies\server.js:95:3)
at null.messageHandler (c:\WebstormProjects\postuj-hovna-server\node_modules\moonridge\node_modules\mongoose\node_modules\mongodb\node_modules\mongodb-core\lib\topologies\server.js:243:23)
at Socket.<anonymous> (c:\WebstormProjects\postuj-hovna-server\node_modules\moonridge\node_modules\mongoose\node_modules\mongodb\node_modules\mongodb-core\lib\connection\connection.js:262:22)
at emitOne (events.js:77:13)
at Socket.emit (events.js:169:7)
at readableAddChunk (_stream_readable.js:146:16)
at Socket.Readable.push (_stream_readable.js:110:10)
at TCP.onread (net.js:521:20)