diff --git a/mmseg/datasets/basesegdataset.py b/mmseg/datasets/basesegdataset.py
index 67760841ed..e97f8ca9d1 100644
--- a/mmseg/datasets/basesegdataset.py
+++ b/mmseg/datasets/basesegdataset.py
@@ -179,7 +179,7 @@ def get_label_map(cls,
                     f'subset of classes {old_classes} in METAINFO.')
             for i, c in enumerate(old_classes):
                 if c not in new_classes:
-                    label_map[i] = -1
+                    label_map[i] = 255
                 else:
                     label_map[i] = new_classes.index(c)
             return label_map