diff --git a/java/src/org/broadinstitute/sting/gatk/walkers/vcf/VCFCombine.java b/java/src/org/broadinstitute/sting/gatk/walkers/vcf/VCFCombine.java
index c4ae8688d..3d32ef27d 100755
--- a/java/src/org/broadinstitute/sting/gatk/walkers/vcf/VCFCombine.java
+++ b/java/src/org/broadinstitute/sting/gatk/walkers/vcf/VCFCombine.java
@@ -116,7 +116,7 @@ public class VCFCombine extends RodWalker<VCFRecord, VCFWriter> {
 
     private void validateAnnotateUnionArguments(String[] priority) {
         Set<RMDTrack> rods = VCFUtils.getRodVCFs(getToolkit());
-        if ( rods.size() != priority.length ) {
+        if ( priority == null || rods.size() != priority.length ) {
             throw new StingException("A complete priority list must be provided when annotateUnion is provided");
         }
         if ( priority.length != 2 ) {