Invalid command line: Cannot process the provided BAM/CRAM file(s)

I am trying to generate a joint call VCF file from two bam files. Here is my code : java -jar /usr/bin/GATK/GenomeAnalysisTK.jar -T HaplotypeCaller -R ucsc.hg19.fasta -I 1-A10_S10.bam -I 1-A11_S11.bam -o jointcalls_A.vcf
But it shows that "ERROR MESSAGE: Invalid command line: Cannot process the provided BAM/CRAM file(s) because they were not indexed."