Why do I get a 'failed to load asset bundle' at editor time?

Now, I had this working fine a few days back but I’ve been adding code and suddenly things aren’t working. I understand that I need to use the isDone event whilst using a script which uses ExecuteInEditMode.

The problem is that when I go into edit mode the download seems to happen an it reaches the isDone event, however download.assetBundle returns null. When I try to instatiate a the asset bundle’s main asset I get an error- failed to load asset bundle.

The asset bundle exists as I am succeffully loading the same assetbundle while in playback mode.

if (download.isDone)
		{
//code seems to get here okay!
		if(download.assetBundle==null)
		{
			//looks like download.assetBundle is null
			print("download.assetBundle didnt exist!!!??!!!");
			download=null;
			return;
		}
//this is where I want to instantiate the assetBundle.mainAsset but I get an error 
//(failed to load asset bundle)

   }

Any ideas?
Many thanks!

DOh, it was my fault. I was pointing at the wrong url. Apologies.