EventSystems throws an error (Missing reference)

Hi, I have a problem. I have Unity 5.1.1 and when I am trying to have “using UnityEngine.EventSystems” the reference is missing. I am trying to use it to get the object being dropped on ondrop function. I couldn’t find a lot of info about this issue. Does anyone know how to fix that?
Here is the code:

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
using UnityEngine.EventSystems;
public class Slot : MonoBehaviour {
    public int slotIndex;
    public Image icon;
    // Use this for initialization
    void Start () {
        icon=this.gameObject.GetComponent<Image>();
    }
 
    // Update is called once per frame
    void Update () {
        if(Control.programSlots[slotIndex]==ProgramRunner.forLoop)
        {
            icon.color=Color.blue;
        }
        if(Control.programSlots[slotIndex]==ProgramRunner.moveBack)
        {
            icon.color=Color.red;
        }
        if(Control.programSlots[slotIndex]==ProgramRunner.moveForward)
        {
            icon.color=Color.yellow;
        }
        if(Control.programSlots[slotIndex]==ProgramRunner.moveLeft)
        {
            icon.color=Color.black;
        }
        if(Control.programSlots[slotIndex]==ProgramRunner.moveRight)
        {
            icon.color=Color.green;
        }
        if(Control.programSlots[slotIndex]==ProgramRunner.blankAction)
        {
            icon.color=Color.white;
        }

    }
    public void OnDrop(EventSystems.PointerEventData data)
    {
        Control.programSlots[slotIndex]=Control.selectedCard.action;
        Control.requestSlots[slotIndex]=Control.selectedCard.req;
    }
}

And here is the error: Assets/Slot.cs(41,28): error CS0246: The type or namespace name `EventSystems’ could not be found. Are you missing a using directive or an assembly reference?

Help Please!

You’re already using EventSystems, so just change your line#41 to this:

public void OnDrop(PointerEventData data)

or if you wish so:

public void OnDrop(UnityEngine.EventSystems.PointerEventData data)