Why did I get an empty vcf file after using GATK's indel caller?

This is the code:
java -jar /Users/RyanYeo/Desktop/GenomeAnalysisTK-3.7/GenomeAnalysisTK.jar -R /Users/RyanYeo/Desktop/hg38Patch7.fa -T UnifiedGenotyper -I /Users/RyanYeo/Desktop/outputwithreadgroups.bam -glm INDEL -o /Users/RyanYeo/Desktop/outputvcf.vcf

Thanks for the help!