error - PDAL Reader.las and Writer.las warning: 'found invalid value of '0' for point's return number'

I am trying to read and write LAS (*.las) file using PDAL but getting the following warning messages by PDAL's LAS reader and writer for ASPRS LAS files.

(pdal pipeline readers.las Warning) C:/denoise\lake.las: 
      Found invalid value of '0' for point's return number.

(pdal pipeline readers.las Warning) C:/denoise\lake.las: 
      Found invalid value of '0' for point's number of returns.

I'm trying to remove noise from las file using the code below:

 { "pipeline":[ "C:/denoise/lake1.las", { "type": "filters.outlier", "method": "statistical", "multiplier": 3, "mean_k": 8 }, { "type": "filters.range", "limits": "Classification![7:7],Z[-100:3000]" }, { "type": "writers.las", "compression": "true", "minor_version": "2", "dataformat_id": "0", "filename":"C:/denoise/clean.las" } ] }