can't use EventSystems, Reference is missing

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 UnityEngine.EventSystems so all the types are exposed from that root level of that namespace, in the parameters/arguments of OnDrop remove the “EventSystems.” from the parameter type like this:

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