The variable has not been assigned error ?

I want to display the name of object along with some other text like, when i mouse over a coke can, a text saying " You are selecting coke" should be displayed. I tried it hard coding, by giving it label and then mouseover and it worked.

But i want a code in which the object name should be assigned during run time like, if mouse over overs a coke can it should say "You are selcting coke" and when its diet coke it should say " You are selecting diet coke"

I tried writing a code, it says "The variable textObject of label_diet has not been assigned" can anyone say why the error ?

 using UnityEngine;
 using System.Collections;
 
 
 public class label_diet : MonoBehaviour {
 
     public GUIText textObject;
 
 
 
     public bool showGUI;
 
 
     void OnMouseOver()
 {
         textObject.text = gameObject.name;
     showGUI = true;
 
 
     
     
 }
 
 
 void OnMouseExit()
 {
     showGUI = false;
 }
 
 
 void OnGUI()
 {
     if (showGUI)
     {
             GUI.Label(new Rect(Input.mousePosition.x,Input.mousePosition.y,100f,100f), "You are selecting Diet coke"+textObject.text);
     }
 }
 
 
 }