diff --git a/pytorch_lightning/trainer/supporters.py b/pytorch_lightning/trainer/supporters.py
index b7a2ec72b4..59e57f2b3a 100644
--- a/pytorch_lightning/trainer/supporters.py
+++ b/pytorch_lightning/trainer/supporters.py
@@ -11,8 +11,6 @@ class TensorRunningMean(object):
         >>> accum.last(), accum.mean()
         (None, None)
         >>> accum.append(torch.tensor(1.5))
-        >>> accum.last(), accum.mean()
-        (tensor(1.5000), tensor(1.5000))
         >>> accum.append(torch.tensor(2.5))
         >>> accum.last(), accum.mean()
         (tensor(2.5000), tensor(2.))
@@ -29,7 +27,7 @@ class TensorRunningMean(object):
         self.rotated: bool = False
 
     def reset(self) -> None:
-        self = TensorRunningMean(self.window_length)
+        self.memory = TensorRunningMean(self.window_length) * 0
 
     def last(self):
         if self.last_idx:
@@ -55,7 +53,4 @@ class TensorRunningMean(object):
             self.rotated = True
 
     def mean(self):
-        if self.last_idx is None:
-            return None
-        avg = self.memory.mean() if self.rotated else self.memory[:self.current_idx].mean()
-        return avg
+        return self.memory.mean()