I'm trying to calculate and export the static of an image (error:Unknown element type provided)

var imagem =ee.ImageCollection("LANDSAT/LC08/C01/T1_RT_TOA")

imagem=imagem.filterBounds(region)                          
    .filterDate ( '2019-01-01' , '2019-12-31' )   
    .map(maskClouds)  

var mean=imagem.mean();

mean=mean.select(['B1', 'B2', 'B3', 'B4', 'B5', 'B6', 'B7']);

var statistic_combine= mean.reduceRegion({

  reducer: ee.Reducer.min()    
                      .combine(ee.Reducer.max(), null, true)    
                      .combine(ee.Reducer.mean(), null, true)    
                      .combine(ee.Reducer.stdDev(), null, true)    
                      .combine(ee.Reducer.median(), null, true)    
                      .combine(ee.Reducer.variance(), null, true),

  geometry: region.geometry(),

  scale: 30,

  maxPixels: 1e9
});

Export.table.toDrive({
    collection: statistica_combine,
    description: 'statistica_table',
    folder: 'GOOGLE_GEE_DIR',
    fileFormat: 'CSV'
  });

---

{Error:Unknown element type provided: object. Expected: ee.Image,
ee.ImageCollection, ee.FeatureCollection or ee.Element}