diff --git a/conf/modules.config b/conf/modules.config
index 69b7967c..cbc04cd0 100644
--- a/conf/modules.config
+++ b/conf/modules.config
@@ -451,7 +451,7 @@ process {
     }
 
     withName: VSEARCH_CLUSTER {
-        ext.args = '--id ${params.vsearch_cluster_id} --usersort --qmask "none"'
+        ext.args = "--id ${params.vsearch_cluster_id} --usersort --qmask 'none'"
         ext.args2 = '--cluster_smallmem'
         ext.args3 = '--clusters'
     }