for my $oligo_indice ( 0 .. $#{ $self->{oligo_ref} } ) {
        # INSERT debug code below - this must be around line 474 :
        for my $j (0 ..  $#{ $self->{hd_matrix_ref}->[$oligo_indice] }){
            die "problem with hd_matrix_ref index '$oligo_indice/$j'" unless defined $self->{hd_matrix_ref}->[$oligo_indice][$j];
        }
        # END insert debug code
        $hd_sum[$oligo_indice] =
            List::Util::sum( @{ $self->{hd_matrix_ref}->[$oligo_indice] } );
    }