diff --git a/cygnus-common/src/main/java/com/telefonica/iot/cygnus/backends/mongo/MongoBackendImpl.java b/cygnus-common/src/main/java/com/telefonica/iot/cygnus/backends/mongo/MongoBackendImpl.java
index 14664d212..9b263e01e 100644
--- a/cygnus-common/src/main/java/com/telefonica/iot/cygnus/backends/mongo/MongoBackendImpl.java
+++ b/cygnus-common/src/main/java/com/telefonica/iot/cygnus/backends/mongo/MongoBackendImpl.java
@@ -215,6 +215,9 @@ public void createIndex(MongoDatabase db, String collectionName, BasicDBObject k
         try {
             db.getCollection(collectionName).createIndex(keys, options);
         } catch(Exception e) {
+            // Our guess is:
+            // IndexOptionsConflict -> when the same index (name and keys) already exits
+            // IndexKeySpecsConflict -> when an index with the same name but different keys (i.e. DM was changed) already exist
             if (e.getMessage().contains("IndexOptionsConflict") ||
                 e.getMessage().contains("IndexKeySpecsConflict")) {
                 db.getCollection(collectionName).dropIndex(options.getName());