diff --git a/data/src/main/java/one/nem/lacerta/data/LacertaLibrary.java b/data/src/main/java/one/nem/lacerta/data/LacertaLibrary.java
index d9be07fe..22279cdd 100644
--- a/data/src/main/java/one/nem/lacerta/data/LacertaLibrary.java
+++ b/data/src/main/java/one/nem/lacerta/data/LacertaLibrary.java
@@ -54,5 +54,5 @@ public interface LacertaLibrary {
 //
 //    CompletableFuture<Void> uncombineDocument(String parentId, ArrayList<String> childIdList);
 
-    CompletableFuture<ArrayList<DocumentDetail>> getCombinedDocumentList(String parentId);
+    CompletableFuture<ArrayList<String>> getCombinedDocumentIdList(String parentId);
 }
diff --git a/data/src/main/java/one/nem/lacerta/data/impl/LacertaLibraryImpl.java b/data/src/main/java/one/nem/lacerta/data/impl/LacertaLibraryImpl.java
index 39a4221d..5b54fd93 100644
--- a/data/src/main/java/one/nem/lacerta/data/impl/LacertaLibraryImpl.java
+++ b/data/src/main/java/one/nem/lacerta/data/impl/LacertaLibraryImpl.java
@@ -341,8 +341,8 @@ public class LacertaLibraryImpl implements LacertaLibrary {
     }
 
     @Override
-    public CompletableFuture<ArrayList<DocumentDetail>> getCombinedDocumentList(String parentId) {
-        return null;
+    public CompletableFuture<ArrayList<String>> getCombinedDocumentIdList(String parentId) {
+
     }
 
     /**