How to solve The input does not contain any JSON tokens Exception?

Question

 var Task1= myRepo.GetList1(data); 

var Task2 = myRepo.GetList2(data);

                await Task.WhenAll(Task1, Task2 );

This line throws an Exception below
            The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. Path: $ | LineNumber: 0 | BytePositionInLine: 0.

What is the reson for this Exception and how to overcome that?