db.u.drop() db.u.insert({x:1}) var x = db.u.findOne() Object .seal(x) throws Thu Jan 17 12:31:05.156 javascript execution failed (shell):1 TypeError: Cannot call method 'isConfigurable' of undefined Object .seal(x) ^ Note: simply accessing each property of the document or iterating it before sealing is enough to prevent the error from occuring. db.u.drop() db.u.insert( {x:1} ) var x = db.u.findOne() for (var p in x) {} Object.seal ; // no longer throws