diff --git a/vardict.wdl b/vardict.wdl
index a421fd785abb9f5786cb0c9781f294c5525c60fd..4d80738a0030b9c3ccd75286463500f378c4aa74 100644
--- a/vardict.wdl
+++ b/vardict.wdl
@@ -37,6 +37,7 @@ task VarDict {
         -G ~{refFasta} \
         -N ~{tumorSampleName} \
         -b "~{tumorBam}~{"|" + normalBam}" \
+        ~{true="-z" false="" defined(normalBam)} \
         -c ~{chromosomeColumn} \
         -S ~{startColumn} \
         -E ~{endColumn} \