how can i fix this Really weird error

I have this script so when you hit a trigger attached to a weapon it will display a gui saying press V to equip. this is my script(i know i can use an OnGUI function but i don't know how to center it on the bottom for every game dimension(stand alone, 5:4, 4:3, etc.) if you could tell me how to do that instead that would work too):

var isGUIActive : boolean = false;

function OnTriggerEnter(hit : Collider)
{
    if(hit.CompareTag("Player"))
    {
        isGUIActive = true;
    }
}

function OnTriggerExit (hit : Collider)
{
    if (hit.CompareTag("Player"))
    {
        isGUIActive = false;
    }
}

function Update()
{
    var gui = GameObject.Find("GUI_equip");
    gui.guiTexture.texture = false;

    if(isGUIActive)
    {
         gui.guiTexture.texture = true;
    }
}

After i added this script i got this weird erroer:

InvalidProgramException: Invalid IL code in equip:Update (): IL_0012: castclass 0x01000006

What does that mean? can you tell me what is wrong?

gui.guiTexture.texture is how you set the texture on it

If you want to set it enabled/disabled, use gui.guiTexture.enabled = true;

Alternatively, use gui.active = true;