diff --git a/dhp-common/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/MergeUtils.java b/dhp-common/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/MergeUtils.java
index e018131102..79a12d6308 100644
--- a/dhp-common/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/MergeUtils.java
+++ b/dhp-common/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/MergeUtils.java
@@ -715,7 +715,9 @@ public class MergeUtils {
 	private static String spKeyExtractor(StructuredProperty sp) {
 		return Optional
 			.ofNullable(sp)
-			.map(s -> Joiner.on("||").join(qualifierKeyExtractor(s.getQualifier()), s.getValue()))
+			.map(s -> Joiner.on("||")
+					.useForNull("")
+					.join(qualifierKeyExtractor(s.getQualifier()), s.getValue()))
 			.orElse(null);
 	}