Please could you help me fix this error.
a) GATK version used: version 3.6-0-g89b7209
b) Exact command used:

java -jar -Xmx8G $EBROOTGATK/GenomeAnalysisTK.jar -T RealignerTargetCreator -R "$REF" -I "$sample"_marked.bam -o "$sample"_realignment_targets.list

c) Entire error log:

##### ERROR --
##### ERROR stack trace
java.lang.IllegalArgumentException: Dictionary cannot have size zero
at org.broadinstitute.gatk.utils.MRUCachingSAMSequenceDictionary.<init>(MRUCachingSAMSequenceDictionary.java:62)
at org.broadinstitute.gatk.utils.GenomeLocParser$1.initialValue(GenomeLocParser.java:78)
at org.broadinstitute.gatk.utils.GenomeLocParser$1.initialValue(GenomeLocParser.java:75)
at java.lang.ThreadLocal.setInitialValue(ThreadLocal.java:180)
at java.lang.ThreadLocal.get(ThreadLocal.java:170)
at org.broadinstitute.gatk.utils.GenomeLocParser.getContigInfo(GenomeLocParser.java:91)
at org.broadinstitute.gatk.utils.GenomeLocParser.getContigs(GenomeLocParser.java:204)
at org.broadinstitute.gatk.utils.GenomeLocParser.<init>(GenomeLocParser.java:135)
at org.broadinstitute.gatk.utils.GenomeLocParser.<init>(GenomeLocParser.java:108)
at org.broadinstitute.gatk.utils.GenomeLocSortedSet.createSetFromSequenceDictionary(GenomeLocSortedSet.java:421)
at org.broadinstitute.gatk.engine.datasources.reads.BAMScheduler.createOverMappedReads(BAMScheduler.java:66)
at org.broadinstitute.gatk.engine.datasources.reads.IntervalSharder.shardOverMappedReads(IntervalSharder.java:55)
at org.broadinstitute.gatk.engine.datasources.reads.SAMDataSource.createShardIteratorOverMappedReads(SAMDataSource.java:1217)
at org.broadinstitute.gatk.engine.GenomeAnalysisEngine.getShardStrategy(GenomeAnalysisEngine.java:649)
at org.broadinstitute.gatk.engine.GenomeAnalysisEngine.execute(GenomeAnalysisEngine.java:307)
at org.broadinstitute.gatk.engine.CommandLineExecutable.execute(CommandLineExecutable.java:113)
at org.broadinstitute.gatk.utils.commandline.CommandLineProgram.start(CommandLineProgram.java:255)
at org.broadinstitute.gatk.utils.commandline.CommandLineProgram.start(CommandLineProgram.java:157)
at org.broadinstitute.gatk.engine.CommandLineGATK.main(CommandLineGATK.java:108)
##### ERROR ------------------------------------------------------------------------------------------
##### ERROR A GATK RUNTIME ERROR has occurred (version 3.6-0-g89b7209):
##### ERROR
##### ERROR This might be a bug. Please check the documentation guide to see if this is a known problem.
##### ERROR If not, please post the error message, with stack trace, to the GATK forum.
##### ERROR Visit our website and forum for extensive documentation and answers to
##### ERROR commonly asked questions https://www.broadinstitute.org/gatk
##### ERROR
##### ERROR MESSAGE: Dictionary cannot have size zero