diff --git a/mmdet/core/bbox/samplers/iou_balanced_neg_sampler.py b/mmdet/core/bbox/samplers/iou_balanced_neg_sampler.py
index 62431d6a073c5f02ed6b984b79459498549c5c7b..5e3ffc3e914f09b82b301eb019425f798e72158b 100644
--- a/mmdet/core/bbox/samplers/iou_balanced_neg_sampler.py
+++ b/mmdet/core/bbox/samplers/iou_balanced_neg_sampler.py
@@ -98,6 +98,8 @@ class IoUBalancedNegSampler(RandomSampler):
                 floor_set = set()
                 iou_sampling_set = set(
                     np.where(max_overlaps > self.floor_thr)[0])
+                # for sampling interval calculation
+                self.floor_thr == 0
 
             floor_neg_inds = list(floor_set & neg_set)
             iou_sampling_neg_inds = list(iou_sampling_set & neg_set)