landsat - How to generate a collection of mosaic images by mapping over date list in Google Earth Engine

I want to generate a collection of mosaicked images from Landsat 5 over multiple years. I have checked that the individual images are not null but on returning the mosaic in the code below throws the error:

List (Error) ImageCollection.fromImages: Attempt to create an ImageCollection with non-image elements.

var p1 = ee.Geometry.Point([103.521,13.028]);
var p2 = ee.Geometry.Point([105.622,13.050]);

var Date_Start = ee.Date('2000-05-01');
var Date_End = ee.Date('2007-12-01');
var Date_window = ee.Number(30);


// Create list of dates for time series
var n_months = Date_End.difference(Date_Start,'month').round();
var dates = ee.List.sequence(0,n_months,1);
var make_datelist = function(n) {
  return Date_Start.advance(n,'month');
};
dates = dates.map(make_datelist);
var fnc = function(d1) {
  var start = ee.Date(d1);
  var end = ee.Date(d1).advance(14,'month');

  var date_range = ee.DateRange(start,end);
  var S1 = ee.ImageCollection('LANDSAT/LT5_L1T_TOA') 
    .filterDate(date_range)
    .sort('CLOUD_COVER')
    .filterBounds(p1).first()

  var S2 = ee.ImageCollection('LANDSAT/LT5_L1T_TOA')
    .filterDate(date_range)
    .sort('CLOUD_COVER')
    .filterBounds(p2).first()


  var mosaic = ee.ImageCollection([ee.Image(S1), ee.Image(S2)]).mosaic();      

  return mosaic
};

var list_of_images = dates.map(fnc);
print('list_of_images', list_of_images);
var mt = ee.ImageCollection(list_of_images);
print(mt);

Map.addLayer(mt, {}, 'mt');

ERROR:

List (Error)
    ImageCollection.fromImages: Attempt to create an ImageCollection with non-image elements.

Answer

I was able to solve the issue (with the help of Google Earth Developer Group's folks). Use var mosaic = ee.Image(S1).blend(ee.Image(S2)) to mosaic instead of mosaic() and make sure the dates have non-null images.

Hope this helps to future readers.