diff --git a/gatk.wdl b/gatk.wdl
index 300c2d40f056218be27fe85d31dcb7e017a28e15..9dbdfdd7759f28fe132817e6ae3a1152ce2e6667 100644
--- a/gatk.wdl
+++ b/gatk.wdl
@@ -56,8 +56,8 @@ task BaseRecalibrator {
     File inputBamIndex
     String recalibrationReportPath
     Array[File]+ sequenceGroupInterval
-    Array[File]? knownIndelsSitesVCFs
-    Array[File]? knownIndelsSitesIndices
+    Array[File?]? knownIndelsSitesVCFs
+    Array[File?]? knownIndelsSitesIndices
     File? dbsnpVCF
     File? dbsnpVCFindex
     File refDict