diff --git a/dhp-workflows/dhp-doiboost/src/main/java/eu/dnetlib/doiboost/crossref/Crossref2Oaf.scala b/dhp-workflows/dhp-doiboost/src/main/java/eu/dnetlib/doiboost/crossref/Crossref2Oaf.scala
index d1f6d8613..1adb7465e 100644
--- a/dhp-workflows/dhp-doiboost/src/main/java/eu/dnetlib/doiboost/crossref/Crossref2Oaf.scala
+++ b/dhp-workflows/dhp-doiboost/src/main/java/eu/dnetlib/doiboost/crossref/Crossref2Oaf.scala
@@ -294,7 +294,7 @@ case object Crossref2Oaf {
     }
 
     def getProjectId (nsPrefix:String, targetId:String):String = {
-      "40|$nsPrefix::$targetId"
+      s"40|$nsPrefix::$targetId"
     }