diff --git a/sat_pred/loss.py b/sat_pred/loss.py
index 3cdd2c6..cd5a81d 100644
--- a/sat_pred/loss.py
+++ b/sat_pred/loss.py
@@ -32,6 +32,9 @@ def name(self) -> str:
     def __call__(self, input: torch.Tensor, target: torch.Tensor) -> torch.Tensor:
         """Return loss"""
 
+        target = target.copy()
+        target[target==-1] = float('nan')
+
         loss = 0
 
         for scale in self.scales:
diff --git a/sat_pred/training_module.py b/sat_pred/training_module.py
index 2895c0d..32620a0 100644
--- a/sat_pred/training_module.py
+++ b/sat_pred/training_module.py
@@ -127,12 +127,7 @@ def __init__(
         self.video_plot_t0_times = video_plot_t0_times
         self.video_crop_plots = video_crop_plots
         self.multi_gpu = multi_gpu
-    
-    @staticmethod
-    def _minus_one_to_nan(y: torch.Tensor) -> None:
-        """Replace -1 values in tensor with NaNs in-place"""
-        y[y==-1] = torch.nan
-        
+
     def _calculate_common_losses(
             self, 
             y: torch.Tensor, 
@@ -147,9 +142,11 @@ def _calculate_common_losses(
         
         losses = {}
         
-        mse_loss = torch.nanmean(F.mse_loss(y_hat, y, reduction="none"))
-        mae_loss = torch.nanmean(F.l1_loss(y_hat, y, reduction="none"))
-        ssim_loss = torch.nanmean(1-self.ssim_func(y_hat, y)) # need to maximise SSIM
+        mask = y==-1
+
+        mse_loss = F.mse_loss(y_hat, y, reduction="none")[~mask].mean()
+        mae_loss = F.l1_loss(y_hat, y, reduction="none")[~mask].mean()
+        ssim_loss = (1-self.ssim_func(y_hat, y))[~mask].mean() # need to maximise SSIM
 
         losses = {
                 "MSE": mse_loss,
@@ -205,10 +202,6 @@ def training_step(self, batch, batch_idx: int) -> None | torch.Tensor:
         y_hat = self.model(X)
         del X
 
-        # Replace the -1 (filled) values in y with NaNs 
-        # This operation is in-place
-        self._minus_one_to_nan(y)
-
         losses = self._calculate_common_losses(y, y_hat)
         losses = {f"{k}/train": v for k, v in losses.items()}
 
@@ -237,10 +230,6 @@ def validation_step(self, batch: dict, batch_idx: int):
         X, y = batch
         y_hat = self.model(X)
         del X
-        
-        # Replace the -1 (filled) values in y with NaNs 
-        # This operation is in-place        
-        self._minus_one_to_nan(y)
     
         losses = self._calculate_common_losses(y, y_hat)
         losses.update(self._calculate_val_losses(y, y_hat))