Mongo error on saving empty changes

[igorshapiro]

 MongoError: '$set' is empty. You must specify a field like so: {$set: {<field>: ...}}
     MongoError: '$set' is empty. You must specify a field like so: {$set: {<field>: ...}}
     at Function.MongoError.create 
     ...
     at _combinedTickCallback (internal/process/next_tick.js:131:7)
     at process._tickDomainCallback (internal/process/next_tick.js:218:9)