I am using vcf command like this: (note: vcf file contains structure variations only)

vcf-compare -p a.vcf.gz b.vcf.gz

both files are processed as follow:

vcf-sort -> bgzip -> tabix

I got this error

Use of uninitialized value $out in print at /usr/local/bin/vcf-compare line 858.    
plot , '-' using 1:2 axes x1y2 with lines lt 0 title "GTs total"
         ^

any idea?