diff --git a/onn_core.py b/onn_core.py
index 5c76e04..04b0fd0 100644
--- a/onn_core.py
+++ b/onn_core.py
@@ -589,11 +589,11 @@ class Layer:
             assert self.parents, self
         edges = []
         for parent in self.parents:
-            # TODO: skip over irrelevant nodes (if any)
-            X = values[parent]
-            if not self.unsafe:
-                self.validate_input(X)
-            edges.append(X)
+            if parent in values:
+                X = values[parent]
+                if not self.unsafe:
+                    self.validate_input(X)
+                edges.append(X)
         Y = self._propagate(edges, deterministic)
         if not self.unsafe:
             self.validate_output(Y)
@@ -604,11 +604,11 @@ class Layer:
             assert self.children, self
         edges = []
         for child in self.children:
-            # TODO: skip over irrelevant nodes (if any)
-            dY = values[child]
-            if not self.unsafe:
-                self.validate_output(dY)
-            edges.append(dY)
+            if child in values:
+                dY = values[child]
+                if not self.unsafe:
+                    self.validate_output(dY)
+                edges.append(dY)
         dX = self._backpropagate(edges)
         if not self.unsafe:
             self.validate_input(dX)