diff --git a/mmseg/apis/test.py b/mmseg/apis/test.py
index 1597df6..9728de4 100644
--- a/mmseg/apis/test.py
+++ b/mmseg/apis/test.py
@@ -149,7 +149,7 @@ def multi_gpu_test(model,
             results.append(result)
 
         if rank == 0:
-            batch_size = data['img'][0].size(0)
+            batch_size = len(result)
             for _ in range(batch_size * world_size):
                 prog_bar.update()