diff --git a/torch_geometric/nn/conv/message_passing.py b/torch_geometric/nn/conv/message_passing.py
index 3f254171888b..6fb50a9c0948 100644
--- a/torch_geometric/nn/conv/message_passing.py
+++ b/torch_geometric/nn/conv/message_passing.py
@@ -241,8 +241,8 @@ def __collect__(self, args, edge_index, size, kwargs):
 
         out['index'] = out['edge_index_i']
         out['size'] = size
-        out['size_i'] = size[1] if size[1] is not None else size[0]
-        out['size_j'] = size[0] if size[0] is not None else size[1]
+        out['size_i'] = size[i] if size[i] is not None else size[j]
+        out['size_j'] = size[j] if size[j] is not None else size[i]
         out['dim_size'] = out['size_i']
 
         return out