diff --git a/conf/modules/post_variant_calling.config b/conf/modules/post_variant_calling.config
index ee29a656ce..3354d4671f 100644
--- a/conf/modules/post_variant_calling.config
+++ b/conf/modules/post_variant_calling.config
@@ -17,6 +17,7 @@
 process {
 
     withName: 'GERMLINE_VCFS_CONCAT'{
+        ext.args   = { "-a" }
         ext.when   = { params.concatenate_vcfs }
         publishDir = [
             //specify to avoid publishing, overwritten otherwise