diff --git a/rlpyt/samplers/base.py b/rlpyt/samplers/base.py
index 2d3922dd..a2c55a16 100644
--- a/rlpyt/samplers/base.py
+++ b/rlpyt/samplers/base.py
@@ -52,8 +52,9 @@ def initialize(self, *args, **kwargs):
         raise NotImplementedError
 
     def obtain_samples(self, itr):
+        # type: (int) -> Samples
         """Execute agent-environment interactions and return data batch."""
-        raise NotImplementedError  # type: Samples
+        raise NotImplementedError
 
     def evaluate_agent(self, itr):
         """Run offline agent evaluation, if applicable."""