diff --git a/dpdata/plugins/deepmd.py b/dpdata/plugins/deepmd.py
index 770d2f9b..3fb8a931 100644
--- a/dpdata/plugins/deepmd.py
+++ b/dpdata/plugins/deepmd.py
@@ -136,6 +136,7 @@ def mix_system(self, *system, type_map, **kwargs):
 
     def from_multi_systems(self, directory, **kwargs):
         sys_dir = []
+        assert os.path.isdir(directory), f"{directory} is not a directory!"
         for root, dirs, files in os.walk(directory):
             if (
                 "type_map.raw" in files