mapping(address => MyStruct[]) public structs;

structs[_user].length = 0; // will trigger an error !

function _burn(uint index) internal {

  require(index < array.length);

  array[index] = array[array.length-1];
  delete array[array.length-1];

  // when array.length is set to 0 it doesn't work:

  array.length = array.length - 1; // doesn't work if array.length = 1

  array.length = 0;  // doesn't work

  // when array.length is set to a value > 0 it works:

  array.length = 1;  // works

  array.length = 2; //  works

}