Issue with env.Vault.ObjectFileOperations.GetObjIDOfFile(140);

I know this is somthing stupid I am doing, but I cant workout what..

I just need to get the ObjID of a file using the ID, so I do;

ObjID templateID = env.Vault.ObjectFileOperations.GetObjIDOfFile(4); 

Great, I get what I need.

However, the teplate document got replaced and now is no longer documentID 4, it is 140, so I do;

ObjID templateID = env.Vault.ObjectFileOperations.GetObjIDOfFile(140); 

And now we get a Exception of

"Class1.cs, 1002 (IL:105), Not found. (0x8004000B)"

Is there somthing I am missing with the usage of GetObjIDOfFile()?

Thanks