GenotypeGVCFs requires a reference
I am trying to do joint genotyping from .gvcf files produced separately per sample, through the command line
java -Xmx4g -jar /bio/GenomeAnalysisTK.jar -T GenotypeGVCFs -V CILtrial/CILsamples.gvcf.list -o CILtrial/CILsamples.gvcf
where CILsamples.gvcf.list is a list of 19 .gvcf files as follows
However, I get the following error message "Walker requires a reference but none was provided."
Based on the GenotypeGVCFs command page (https://software.broadinstitute.org/gatk/gatkdocs/org_broadinstitute_gatk_tools_walkers_variantutils_GenotypeGVCFs.php), the only required input is -V, so why do I get this?
Maybe a possible explanation is the one stated in a similar discussion (http://gatkforums.broadinstitute.org/gatk/discussion/7326/asereacounter-requiring-a-reference), i.e. that GATK tools always require a reference..but wouldn't this be discordant with what is stated in the GenotypeGVCFs page?
It may probably be a stupid question,
but I did not manage to find any other plausible answer in the forum.