Previously to 3.2 when this happened var s = [] s [1] = 1 print(s) => [ undefined, 1] db.colll.insert( {mys:s} ) you would get a doc like this: { mys: [ null, 1] } now in v3.2 you get this document: { mys: [1] } So the data has shifted up an index. Is this a new BUG or an old BUG fixed - cannot find any reference to it elsewhere. ==== fyi: It has played havoc with some of my Map Reductions which rely on the specific placement of values in an array against a standard header array which I use to populate the array i.e. value.arr[ header.indexOf( this.term ) ] = this.avalue; emit(..., value) I think that there might be an insert between the map and reduction where the array is restructured.