diff --git a/mmdet/datasets/builder.py b/mmdet/datasets/builder.py
index 1d101ff93e17a24a1193285ee999dc142a56aa11..6e707b1901637aae61379b53fcd00508f9a2982d 100644
--- a/mmdet/datasets/builder.py
+++ b/mmdet/datasets/builder.py
@@ -8,7 +8,7 @@ from .registry import DATASETS
 def _concat_dataset(cfg, default_args=None):
     ann_files = cfg['ann_file']
     img_prefixes = cfg.get('img_prefix', None)
-    seg_prefixes = cfg.get('seg_prefixes', None)
+    seg_prefixes = cfg.get('seg_prefix', None)
     proposal_files = cfg.get('proposal_file', None)
 
     datasets = []