diff --git a/src/kyupy/circuit.py b/src/kyupy/circuit.py
index 089e68e..b1107f6 100644
--- a/src/kyupy/circuit.py
+++ b/src/kyupy/circuit.py
@@ -459,7 +459,7 @@ class Circuit:
         for l, ll in zip(impl_out_lines, node_out_lines):  # connect outputs
             if ll is None:
                 if l.driver in node_map:
-                    self.remove_dangling_nodes(node_map[l.driver], keep=set(self.s_nodes))
+                    self.remove_dangling_nodes(node_map[l.driver], keep=ios)
                 continue
             if len(l.reader.outs) > 0:  # output is also read by impl. circuit, connect to fork.
                 ll.driver = node_map[l.reader]