The error message is:
Attribute (control_signal_for_tissue) does not pass the type constraint because: Validation failed for 'HashRef[Num]' with value %dif_in_control_sig at /home/ray/cxgn/sgn/lib//SGN/Feature/ExpressionViewer/Converter.pm line 145
	SGN::Feature::ExpressionViewer::Converter::calculate_comparison('SGN::Feature::ExpressionViewer::Converter=HASH(0x8bda4a0)', 'SGN::Feature::ExpressionViewer::Converter=HASH(0x8be3610)', 0, 0, 0, 0) called at /home/ray/cxgn/sgn/lib//SGN/Feature/ExpressionViewer/Analyzer.pm line 133
	SGN::Feature::ExpressionViewer::Analyzer::make_comparison_picture('SGN::Feature::ExpressionViewer::Analyzer=HASH(0x8be3540)', 0, 0, 0, 0) called at image_conversion.t line 419

And the code is:
   my ($self, $comparison_converter, $threshold,
                $override, $grey_mask_on, $mask_ratio) = @_;
   my %dif_in_gene_sig;
   my %dif_in_control_sig;
   for my $tissue ($self->tissues)
   {
      my $gene1Sig = $self->gene_signal_in_tissue->{$tissue};
      my $gene1Control = $self->control_signal_for_tissue->{$tissue};
      my $gene2Sig = $comparison_converter->gene_signal_in_tissue->{$tissue};
      my $gene2Control = $comparison_converter->control_signal_for_tissue->{$tissue};
      #If all are defined and not 0,
      if ($gene2Sig and $gene2Control and $gene1Sig and $gene1Control)
      {
         $dif_in_gene_sig{$tissue} = $gene1Sig/$gene2Sig;
         $dif_in_control_sig{$tissue} = $gene1Control/$gene2Control;
         #print
        #"$tissue\t$dif_in_gene_sig{$tissue}\t$dif_in_control_sig{$tissue}\n";
      }
   }
#Line 145# my $temp_gene_analysis =
        SGN::Feature::ExpressionViewer::Converter->new(
           'gene_signal_in_tissue'=>"\%dif_in_gene_sig",
               'control_signal_for_tissue'=>"\%dif_in_control_sig");
   return ($temp_gene_analysis->calculate_relative($threshold, $grey_mask_on,
                                                        $override, $mask_ratio), $temp_gene_analysis->get_min, $temp_gene_analysis->get_median);

It's surprising because when I ran the print command, I got a PO term\tnumber\tnumber\n.