diff --git a/dnet-ariadneplus-graphdb-publisher/src/main/java/eu/dnetlib/ariadneplus/graphdb/GraphDBClient.java b/dnet-ariadneplus-graphdb-publisher/src/main/java/eu/dnetlib/ariadneplus/graphdb/GraphDBClient.java
index 9a8b78c..0edc308 100644
--- a/dnet-ariadneplus-graphdb-publisher/src/main/java/eu/dnetlib/ariadneplus/graphdb/GraphDBClient.java
+++ b/dnet-ariadneplus-graphdb-publisher/src/main/java/eu/dnetlib/ariadneplus/graphdb/GraphDBClient.java
@@ -398,6 +398,9 @@ public class GraphDBClient {
 	public String indexOnESByIdentifier(String datasource, String collectionId, String resourceType, String identifier) throws AriadnePlusPublisherException {
 		String report = "";
 		try {
+			if (StringUtils.isBlank(identifier)) {
+				return "empty identifier";
+			}
 			List <String> identifiers = Arrays.asList(identifier);
 			log.info("Start indexing from "+ datasource + " " + collectionId);
 			runSPQRLQuery.setupConnection( getWriterUser(), getWriterPwd(), this.graphDBServerUrl, getRepository());
diff --git a/dnet-ariadneplus-graphdb-publisher/src/main/java/eu/dnetlib/ariadneplus/reader/RunSPARQLQueryService.java b/dnet-ariadneplus-graphdb-publisher/src/main/java/eu/dnetlib/ariadneplus/reader/RunSPARQLQueryService.java
index 25767d5..e36b0db 100644
--- a/dnet-ariadneplus-graphdb-publisher/src/main/java/eu/dnetlib/ariadneplus/reader/RunSPARQLQueryService.java
+++ b/dnet-ariadneplus-graphdb-publisher/src/main/java/eu/dnetlib/ariadneplus/reader/RunSPARQLQueryService.java
@@ -163,7 +163,11 @@ public class RunSPARQLQueryService {
 			return bulkUpload.index(resourceManager, isCollection);
         } catch (HTTPQueryEvaluationException qe) {
 			log.error(qe);
-			return -5;
+			if (qe.getMessage()!=null &&
+					qe.getMessage().contains("Insufficient free Heap Memory")) {
+				return -5;
+			}
+			return -6;
 		} catch(Exception e){
             log.error(e);
             return -1;