diff --git a/torchcast/exp_smooth/exp_smooth.py b/torchcast/exp_smooth/exp_smooth.py
index 2a3f59c..e67886a 100644
--- a/torchcast/exp_smooth/exp_smooth.py
+++ b/torchcast/exp_smooth/exp_smooth.py
@@ -43,7 +43,6 @@ def _update(self,
         resid = input - measured_mean
         new_mean = mean + (kwargs['K'] @ resid.unsqueeze(-1)).squeeze(-1)
         # _update doesn't waste compute creating new_cov; in predict cov will be replaced by cov1step
-        # TODO: why not replace it here?
         new_cov = torch.tensor(0.0, dtype=mean.dtype, device=mean.device)
         return new_mean, new_cov
 
@@ -57,7 +56,11 @@ def predict(self,
         new_mean = update_tensor(mean, new=(F @ mean[mask].unsqueeze(-1)).squeeze(-1), mask=mask)
         new_cov = kwargs['cov1step']
         if len(cov.shape):  # see note in _update() above
-            new_cov = update_tensor(new_cov.clone(), new=F @ cov[mask] @ F.permute(0, 2, 1), mask=mask)
+            new_cov = update_tensor(
+                orig=new_cov,
+                new=new_cov[mask] + F @ cov[mask] @ F.permute(0, 2, 1),
+                mask=mask
+            )
 
         return new_mean, new_cov