diff --git a/mmdet/models/builder.py b/mmdet/models/builder.py
index bdf0ac3d16f9aadb194f944b3f7c4dd1a741e8cd..ee5ae0b14b01e147f5f9199141709bdac4dbe0af 100644
--- a/mmdet/models/builder.py
+++ b/mmdet/models/builder.py
@@ -2,7 +2,7 @@ from mmcv.runner import obj_from_dict
 from torch import nn
 
 from . import (backbones, necks, roi_extractors, rpn_heads, bbox_heads,
-               mask_heads, detectors)
+               mask_heads)
 
 __all__ = [
     'build_backbone', 'build_neck', 'build_rpn_head', 'build_roi_extractor',
@@ -48,4 +48,5 @@ def build_mask_head(cfg):
 
 
 def build_detector(cfg, train_cfg=None, test_cfg=None):
+    from . import detectors
     return build(cfg, detectors, dict(train_cfg=train_cfg, test_cfg=test_cfg))
diff --git a/mmdet/models/rpn_heads/rpn_head.py b/mmdet/models/rpn_heads/rpn_head.py
index e67d7ae973f05c60c8e226009cfb4234c0894f69..61e6e199ac0407bd23226701e3117c02ec16171d 100644
--- a/mmdet/models/rpn_heads/rpn_head.py
+++ b/mmdet/models/rpn_heads/rpn_head.py
@@ -48,8 +48,8 @@ class RPNHead(nn.Module):
         self.anchor_scales = anchor_scales
         self.anchor_ratios = anchor_ratios
         self.anchor_strides = anchor_strides
-        self.anchor_base_sizes = anchor_strides.copy(
-        ) if anchor_base_sizes is None else anchor_base_sizes
+        self.anchor_base_sizes = list(
+            anchor_strides) if anchor_base_sizes is None else anchor_base_sizes
         self.target_means = target_means
         self.target_stds = target_stds
         self.use_sigmoid_cls = use_sigmoid_cls