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I'm trying to calculate the VCI (NDVI - minNDVI)/(maxNDVI-minNDVI) for a collection of landsat 5 merged with a collection of landsat 8 because I will need to do this for different periods covering from 1990 to 2018. For testing, I'm using a period from 1998 to 2000 for Switzerland. If I run the following code, I run into the error:

VCI image: Tile error: Expected a homogeneous image collection, but an image with an incompatible band was encountered. Mismatched type for band 'NDVI': Expected type: Float<-0.29947625586281806, 1.2251065502612928>. Actual type: Float<-0.4166935481920486, 1.2483430624070735>. Image ID: null This band might require an explicit cast.

Is there a problem with how I calculate the NDVI? How could I solve this error?

var bbox = ee.Geometry.Polygon([
  [5.9559,45.818],
  [5.9559,47.8084],
  [10.4921,47.8084],
  [10.4921,45.818],
  [5.9559,45.818]]);
  
Map.centerObject(bbox);
Map.addLayer(bbox, {color: 'FF0000'}, 'geodesic polygon');

// Assign a common name to the sensor-specific bands.
var LC8_BANDS = ['B2',   'B3',    'B4',  'B5',  'B6',    'B7',    'B10']; //Landsat 8
var LC5_BANDS = ['B1',   'B2',    'B3',  'B4',  'B5',    'B7',    'B6']; //Llandsat 5
var STD_NAMES = ['blue', 'green', 'red', 'nir', 'swir1', 'swir2', 'temp'];

var l8 = ee.ImageCollection('LANDSAT/LC08/C01/T1_TOA').filterDate('1998-1-1', '2000-12-31')
                                                      .filterBounds(bbox);// Landsat 8
//print(l8, 'Landsat 8')

var l5 = ee.ImageCollection('LANDSAT/LT05/C01/T1_TOA').filterDate('1998-1-1', '2000-12-31')
                                                      .filterBounds(bbox); //Landsat 5
//print(l5, 'Landsat 5')

var withCloudiness = l8.map(function(image) {
  var cloud = ee.Algorithms.Landsat.simpleCloudScore(image).select('cloud');
  var cloudiness = cloud.reduceRegion({
    reducer: 'mean', 
    geometry: bbox, 
    bestEffort: true,
    scale: 30,
  });

  return image.set(cloudiness);
});

var filteredCollectionl8 = withCloudiness.filter(ee.Filter.lt('cloud', 20)).select(LC8_BANDS, STD_NAMES);

var withCloudiness = l5.map(function(image) {
  var cloud = ee.Algorithms.Landsat.simpleCloudScore(image).select('cloud');
  var cloudiness = cloud.reduceRegion({
    reducer: 'mean', 
    geometry: bbox, 
    bestEffort: true,
    scale: 30,
  });

  return image.set(cloudiness);
});

var filteredCollectionl5 = withCloudiness.filter(ee.Filter.lt('cloud', 20)).select(LC5_BANDS, STD_NAMES);

var lall = ee.ImageCollection(filteredCollectionl5.merge(filteredCollectionl8));
//print(lall, 'Merged')

var addNDVI = function(image) {
  var ndvi = image.normalizedDifference(['nir', 'red']).rename('NDVI');
  return image.addBands(ndvi);
};

var ndviColl = lall.map(addNDVI);

var vci = ndviColl.map(function(img){
 var id = img.id();
 var min =  img.reduceRegion(ee.Reducer.min(), bbox,300).get('NDVI');
 var max = img.reduceRegion(ee.Reducer.max(), bbox,300).get('NDVI');
 return img.expression(
   "(NDVI-min)/(max-min)",{
     "NDVI" : img,
     "max" : ee.Number(max),
     "min" : ee.Number(min)
   }).copyProperties(img,['system:time_start','system:time_end']);
});

print(vci, 'VCI')

// Display the result.
//Map.centerObject(vci, 9);
var ndviParams = {min: -1, max: 1, palette: ['blue', 'white', 'green']};
var vciParams = {min: 0, max:1}
Map.addLayer(vci.mean(), vciParams, 'VCI image');
0

That error means that not all bands in your ImageCollection are of the same type. Try casting them to float:

var addNDVI = function(image) {
  var ndvi = image.normalizedDifference(['nir', 'red']).rename('NDVI');
  return image.addBands(ndvi).toFloat();
};

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