Variant Recalibrator 0 scoring variants

Hi,

We've been experiencing an error with the Variant Recalibrator tool in a number of our runs. The error occurred with versions 3.5 and 3.7, with WGS data that shouldn't break the tool due to an insufficient number of variants. The stack trace that we get looks like this:

INFO  15:08:58,235 VariantRecalibratorEngine - Evaluating full set of 4571033 variants... ``
INFO  15:08:58,325 VariantDataManager - Training with worst 0 scoring variants --> variants with LOD <= -5.0000. 
##### ERROR --
##### ERROR stack trace 
org.broadinstitute.gatk.utils.exceptions.ReviewedGATKException: Unable to retrieve result
    at org.broadinstitute.gatk.engine.executive.HierarchicalMicroScheduler.execute(HierarchicalMicroScheduler.java:190)
    at org.broadinstitute.gatk.engine.GenomeAnalysisEngine.execute(GenomeAnalysisEngine.java:316)
    at org.broadinstitute.gatk.engine.CommandLineExecutable.execute(CommandLineExecutable.java:123)
    at org.broadinstitute.gatk.utils.commandline.CommandLineProgram.start(CommandLineProgram.java:256)
    at org.broadinstitute.gatk.utils.commandline.CommandLineProgram.start(CommandLineProgram.java:158)
    at org.broadinstitute.gatk.engine.CommandLineGATK.main(CommandLineGATK.java:108)
Caused by: java.lang.IllegalArgumentException: No data found.
    at org.broadinstitute.gatk.tools.walkers.variantrecalibration.VariantRecalibratorEngine.generateModel(VariantRecalibratorEngine.java:88)
    at org.broadinstitute.gatk.tools.walkers.variantrecalibration.VariantRecalibrator.onTraversalDone(VariantRecalibrator.java:489)
    at org.broadinstitute.gatk.tools.walkers.variantrecalibration.VariantRecalibrator.onTraversalDone(VariantRecalibrator.java:185)
    at org.broadinstitute.gatk.engine.executive.HierarchicalMicroScheduler.notifyTraversalDone(HierarchicalMicroScheduler.java:226)
    at org.broadinstitute.gatk.engine.executive.HierarchicalMicroScheduler.execute(HierarchicalMicroScheduler.java:183)
    ... 5 more
##### ERROR ------------------------------------------------------------------------------------------
##### ERROR A GATK RUNTIME ERROR has occurred (version 3.7-0-gcfedb67):
##### 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://software.broadinstitute.org/gatk
##### ERROR
##### ERROR MESSAGE: Unable to retrieve result
##### ERROR ------------------------------------------------------------------------------------------

Thank you for your help!