javascript - Uncaught TypeError: Cannot set property 'onclick' of null

I'm having problems making my window alert pop up with a simple checkbox and can't for the life of me figure out why. Here's the basic Javascript and HTML:

var blue_box=document.getElementById("color-blue");
function colorFunction() {
    window.alert("This color is blue!");
}

blue_box.onclick=colorFunction;

    

        
 Form!
        


            
                    name="color" 
                    value="blue" 
                    id="color-blue" />
                    blue
                
	        
                    name="color"  
                    value="red" 
                    id="color-red" />

                    red
                
		
                    name="color" 
                    value="yellow" 
                    id="color-yellow" />
                    yellow 
                
            
        

    

Which throws: Uncaught TypeError: Cannot set property 'onclick' of null
under

blue_box.onclick=colorFunction;

Are there any visible reasons for this error in my code?