diff --git a/graphrag/mind_map_extractor.py b/graphrag/mind_map_extractor.py
index fabdaed08..652c871d6 100644
--- a/graphrag/mind_map_extractor.py
+++ b/graphrag/mind_map_extractor.py
@@ -105,6 +105,9 @@ class MindMapExtractor:
             for i, _ in enumerate(threads):
                 res.append(_.result())
 
+            if not res:
+                return MindMapResult(output={"root":{}})
+
             merge_json = reduce(self._merge, res)
             if len(merge_json.keys()) > 1:
                 keyset = set(