diff --git a/src/main/java/org/apache/commons/collections4/multimap/HashSetValuedHashMap.java b/src/main/java/org/apache/commons/collections4/multimap/HashSetValuedHashMap.java
index de891b355..a68f1e34b 100644
--- a/src/main/java/org/apache/commons/collections4/multimap/HashSetValuedHashMap.java
+++ b/src/main/java/org/apache/commons/collections4/multimap/HashSetValuedHashMap.java
@@ -130,6 +130,12 @@ public class HashSetValuedHashMap<K, V> extends AbstractSetValuedMap<K, V>
         doReadObject(in);
     }
 
+    /**
+     * Serializes this object to an ObjectOutputStream.
+     *
+     * @param out the target ObjectOutputStream.
+     * @throws IOException thrown when an I/O errors occur writing to the target stream.
+     */
     private void writeObject(final ObjectOutputStream out) throws IOException {
         out.defaultWriteObject();
         doWriteObject(out);