ImageBtn has no properties

//This is the callback from AJAX
function ParseBuildingInfo()
{
    //readyState of 4 or 'complete' represents that data has been returned
    if (objXmlHttp.readyState == 4 || objXmlHttp.readyState == 'complete')
    {
		var strUpgrade = objXmlHttp.responseText.split( '\n' );
		
		for( i=0; i<strUpgrade.length; i+=6 )
		{
			var ID_arr = i / 8;
			i++;
	
			showdiv( "Building_Upgrades" );	
			
			// The array IDs contains all the IDs after ASP.NET is done making them unique.
			var ImageBtn = document.getElementById( IDs[ID_arr] );
			
			ImageBtn.title = strUpgrade[i+1] + '\n' + "W:" + strUpgrade[i+2] +
				"  I:" + strUpgrade[i+3] + "  S:" + strUpgrade[i+4] + "  G:" + strUpgrade[i+5] + " Time:" + strUpgrade[i+6]; 
			ImageBtn.src = strUpgrade[i+1];
		}
		
    }
}

//This is how I am hiding my . Show div is the same except it uses "block"
function hidediv(id) 
{
	//safe function to hide an element with a specified id
	if (document.getElementById) 
	{ 
		// DOM3 = IE5, NS6
		document.getElementById(id).style.display = 'none';
	}
	else 
	{
		if (document.layers)
		{ 
			// Netscape 4
			document.id.display = 'none';
		}
		else
		{ 
			// IE 4
			document.all.id.style.display = 'none';
		}
	}
}