Postgis DB to mbtile conversion - coordinate system issue!

I am trying to run below command.

tl copy "postgis://username:password@host_id:5432/test?table=schema.lines&geometry_field=geom&srid=3857&layerName=road" "mbtiles://./belgium-tiles.mbtiles" -z 1 -Z 22 -b "2 49 6 52"

But while executing it, I am getting below error:

"C:\Program Files\JetBrains\WebStorm 2017.2.5\bin\runnerw.exe" "C:\Program Files\nodejs\node.exe" C:\Users\bankard\AppData\Roaming\npm\node_modules\tl\bin\tl.js copy postgis://journaldb:journaldb@insrvut-cpp01:5433/raah?table=tileserver_test.lines&geometry_field=geom&srid=3857&layerName=road mbtiles://./belgium-tiles.mbtiles -z 1 -Z 22 -b "2 49 6 52"
C:\Users\bankard\AppData\Roaming\npm\node_modules\tl\node_modules\tilelive-streaming\lib\scanlinescheme.js:45
    if (options.bbox[0] > options.bbox[2]) throw new Error('bbox is invalid');
                                           ^

Error: bbox is invalid
    at new ScanlineScheme (C:\Users\bankard\AppData\Roaming\npm\node_modules\tl\node_modules\tilelive-streaming\lib\scanlinescheme.js:45:50)
    at Function.Scheme.create (C:\Users\bankard\AppData\Roaming\npm\node_modules\tl\node_modules\tilelive-streaming\lib\scheme.js:52:12)
    at C:\Users\bankard\AppData\Roaming\npm\node_modules\tl\node_modules\tilelive-streaming\index.js:161:21
    at Immediate.<anonymous> (C:\Users\bankard\AppData\Roaming\npm\node_modules\tl\node_modules\locking-cache\index.js:73:25)
    at runCallback (timers.js:672:20)
    at tryOnImmediate (timers.js:645:5)
    at processImmediate [as _immediateCallback] (timers.js:617:5)

While debugging I have found that
in file tilelive-streaming/index.js

  var restrict = function(info, by) {
  info = applyConfigDefaults(info);
  by = applyConfigDefaults(by);

  // restrict the options according to known restrictions
  info.minzoom = Math.max(info.minzoom, by.minzoom);
  info.maxzoom = Math.min(info.maxzoom, by.maxzoom);
  info.bounds[0] = Math.max(info.bounds[0], by.bounds[0]);
  info.bounds[1] = Math.max(info.bounds[1], by.bounds[1]);
  info.bounds[2] = Math.min(info.bounds[2], by.bounds[2]);
  info.bounds[3] = Math.min(info.bounds[3], by.bounds[3]);

  return info;
};

this code is actually changing bounding boxes. and replacing it by bounding boxes provided in by object. Can anybody help me to understand from where it is actually computing by 's value and what am I doing wrong in sending parameters?

Start with tilelive-streaming/index.js, especially restrict, and trace the copy command into scheme creation in tilelive-streaming/lib/scanlinescheme.js. Follow how the by options are built from the supplied -b and source bounds, then reproduce the command and document or test the expected bounding-box validation behavior.