Raycast not working ?

Hello there !

I'm having a problem with Raycasting. I'm using NGUI and I'm trying to implement some universal OnMouse(Enter/Exit/...)-like callback, but no matter what I try, I can't get it working. Here's the full code :

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent(typeof(Collider))]
 public class cpTip : MonoBehaviour {
 
     // Use this for initialization
     void Start () 
     {
         
     }
     
     // Update is called once per frame
     void Update () 
     {
         //Debug.Log("Update()");
         Camera cam;
         Color color;
         if ( gameObject.GetComponent<UIWidget>() != null)
         {
             cam = UICamera.currentCamera;
             color = Color.red;
             Debug.Log(gameObject.name + " : UI Camera");
         }
         else
         {
             cam = Camera.mainCamera;
             color = Color.blue;
             Debug.Log(gameObject.name + " : Main Camera");
         }
         Ray ray = cam.ScreenPointToRay(Input.mousePosition);
         RaycastHit hit;
         if ( Physics.Raycast(ray, out hit, 100, gameObject.layer ) )
         {
             Debug.Log("Mouse over object");
         }
         Debug.DrawLine( ray.origin, ray.origin+ray.direction*10, color, 0, true);
     }
     
     void OnMouseOver()
     {
         //Debug.Log("OnMouseOver()");
     }
 }

Can someone tell me what I'm doing wrong here ?

Thanks in advance :)