diff --git a/Runtime/Scripts/Core/Cluster.cs b/Runtime/Scripts/Core/Cluster.cs
index 7dcfafb..18332e1 100644
--- a/Runtime/Scripts/Core/Cluster.cs
+++ b/Runtime/Scripts/Core/Cluster.cs
@@ -387,7 +387,7 @@ namespace NanoBrain {
             return allInstances;
         }
 
-        public void Backpropagation(Func<Cluster, Vector3> Observer, Vector3 target, float learningRate) {
+        public void Backpropagation(Func<Cluster, float> Observer, float target, float learningRate) {
             foreach (Nucleus nucleus in this.instances[0].nuclei) {
                 if (nucleus is not Neuron neuron)
                     continue;