diff --git a/dhp-workflows/dhp-aggregation/src/main/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIterator.java b/dhp-workflows/dhp-aggregation/src/main/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIterator.java
index 46401a5b5..579ef6127 100644
--- a/dhp-workflows/dhp-aggregation/src/main/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIterator.java
+++ b/dhp-workflows/dhp-aggregation/src/main/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIterator.java
@@ -25,32 +25,47 @@ import eu.dnetlib.dhp.common.aggregation.AggregatorReport;
 
 public class BaseCollectorIterator implements Iterator<Element> {
 
-	private final BlockingQueue<Element> queue = new LinkedBlockingQueue<>();
+	private Object nextElement;
+
+	private final BlockingQueue<Object> queue = new LinkedBlockingQueue<>();
 
 	private static final Logger log = LoggerFactory.getLogger(BaseCollectorIterator.class);
 
-	private boolean completed = false;
-
 	public BaseCollectorIterator(final FileSystem fs, final Path filePath, final AggregatorReport report) {
 		new Thread(() -> importHadoopFile(fs, filePath, report)).start();
+		try {
+			this.nextElement = this.queue.take();
+		} catch (final InterruptedException e) {
+			throw new RuntimeException(e);
+		}
 	}
 
 	protected BaseCollectorIterator(final String resourcePath, final AggregatorReport report) {
 		new Thread(() -> importTestFile(resourcePath, report)).start();
+		try {
+			this.nextElement = this.queue.take();
+		} catch (final InterruptedException e) {
+			throw new RuntimeException(e);
+		}
 	}
 
 	@Override
 	public synchronized boolean hasNext() {
-		return !this.queue.isEmpty() || !isCompleted();
+		return (this.nextElement != null) && (this.nextElement instanceof Element);
 	}
 
 	@Override
 	public synchronized Element next() {
 		try {
-			return this.queue.take();
-		} catch (final InterruptedException e) {
-			throw new RuntimeException(e);
+			return this.nextElement instanceof Element ? (Element) this.nextElement : null;
+		} finally {
+			try {
+				this.nextElement = this.queue.take();
+			} catch (final InterruptedException e) {
+				throw new RuntimeException(e);
+			}
 		}
+
 	}
 
 	private void importHadoopFile(final FileSystem fs, final Path filePath, final AggregatorReport report) {
@@ -64,7 +79,7 @@ public class BaseCollectorIterator implements Iterator<Element> {
 			report.put(e.getClass().getName(), e.getMessage());
 			throw new RuntimeException("Error processing BASE records", e);
 		} finally {
-			setCompleted(true);
+			this.queue.add("__END__"); // I ADD A NOT ELEMENT OBJECT TO INDICATE THE END OF THE QUEUE
 		}
 	}
 
@@ -77,7 +92,7 @@ public class BaseCollectorIterator implements Iterator<Element> {
 			report.put(e.getClass().getName(), e.getMessage());
 			throw new RuntimeException("Error processing BASE records", e);
 		} finally {
-			setCompleted(true);
+			this.queue.add("__END__"); // I ADD A NOT ELEMENT OBJECT TO INDICATE THE END OF THE QUEUE
 		}
 	}
 
@@ -104,7 +119,7 @@ public class BaseCollectorIterator implements Iterator<Element> {
 
 					for (final Object o : doc.selectNodes("//*[local-name()='ListRecords']/*[local-name()='record']")) {
 						if (o instanceof Element) {
-							this.queue.add((Element) o);
+							this.queue.add(o);
 							count++;
 						}
 					}
@@ -114,12 +129,4 @@ public class BaseCollectorIterator implements Iterator<Element> {
 		log.info("Total records (written in queue): " + count);
 	}
 
-	private synchronized boolean isCompleted() {
-		return this.completed;
-	}
-
-	private synchronized void setCompleted(final boolean completed) {
-		this.completed = completed;
-	}
-
 }