diff --git a/andes/io/matpower.py b/andes/io/matpower.py
index 4a8498b90..97c41f38c 100644
--- a/andes/io/matpower.py
+++ b/andes/io/matpower.py
@@ -307,7 +307,7 @@ def _get_bus_id_caller(bus):
 
     """
 
-    if np.array(bus.idx.v).dtype == np.object:
+    if np.array(bus.idx.v).dtype == object:
         return lambda x: bus.idx2uid(x) + 1
     else:
         return lambda x: x