diff --git a/maskrcnn_benchmark/modeling/backbone/resnet.py b/maskrcnn_benchmark/modeling/backbone/resnet.py
index cff6863bc81f0db5b47a05fd507b69471ff37ec3..4366a70a6f37956ab33406fbabfaccc2f6c2ea01 100644
--- a/maskrcnn_benchmark/modeling/backbone/resnet.py
+++ b/maskrcnn_benchmark/modeling/backbone/resnet.py
@@ -34,22 +34,22 @@ StageSpec = namedtuple(
 # Standard ResNet models
 # -----------------------------------------------------------------------------
 # ResNet-50 (including all stages)
-ResNet50StagesTo5 = (
+ResNet50StagesTo5 = tuple(
     StageSpec(index=i, block_count=c, return_features=r)
     for (i, c, r) in ((1, 3, False), (2, 4, False), (3, 6, False), (4, 3, True))
 )
 # ResNet-50 up to stage 4 (excludes stage 5)
-ResNet50StagesTo4 = (
+ResNet50StagesTo4 = tuple(
     StageSpec(index=i, block_count=c, return_features=r)
     for (i, c, r) in ((1, 3, False), (2, 4, False), (3, 6, True))
 )
 # ResNet-50-FPN (including all stages)
-ResNet50FPNStagesTo5 = (
+ResNet50FPNStagesTo5 = tuple(
     StageSpec(index=i, block_count=c, return_features=r)
     for (i, c, r) in ((1, 3, True), (2, 4, True), (3, 6, True), (4, 3, True))
 )
 # ResNet-101-FPN (including all stages)
-ResNet101FPNStagesTo5 = (
+ResNet101FPNStagesTo5 = tuple(
     StageSpec(index=i, block_count=c, return_features=r)
     for (i, c, r) in ((1, 3, True), (2, 4, True), (3, 23, True), (4, 3, True))
 )