diff --git a/graph-neural-operator/utilities.py b/graph-neural-operator/utilities.py
index af44e1a..b32ce81 100644
--- a/graph-neural-operator/utilities.py
+++ b/graph-neural-operator/utilities.py
@@ -493,7 +493,7 @@ def assemble(self, pred, split_idx, batch_size2, sigma=1):
             for j in range(batch_size2):
                 pred_ij = pred_i[j,:].reshape(-1,)
                 idx = split_idx_i[j,:].reshape(-1,)
-                out[idx] = pred_ij
+                out[idx] += pred_ij
 
         out = out / self.l