Calling static jar function from Unity3D

I made and compiled a Android Library, containing a simple class and a simple static function:

 package moo;
 
 public class MyTestClass {
 
     public static String Foo(){
         return "Foo from Moo";
     }
     
 }

I placed the .jar in my Assets/Plugins/Android Folder. Then In Unity:

     void OnGUI () {
         string somestring = "foooooooooooOOooo";
         
         AndroidJavaClass testClass = new AndroidJavaClass("moo.MyTestClass");
         somestring = testClass.CallStatic<string>("Foo");
         
         GUI.Label (new Rect (20, 20, 100, 20), somestring);
     }

And I get an error: JNI: Unable to find method id for 'Foo' (static) UnityEngine.AndroidJavaObject:CallStatic(String, Object[])

Am I missing something to call my static method? Thanks!