The system identifier must begin with either a single or double quote character

I've an XML schema that references the a well-known schema hosted on the web.

http://ift.tt/1ACtnKe

Via:

<xs:import namespace="http://ift.tt/1ikcaie" schemaLocation="http://ift.tt/1ACtnKe"/>

Via dependencies I haven't chased down, it references this:

http://ift.tt/1ACto0w

Which has a validation error according to Xerces, saying:

The system identifier must begin with either a single or double quote character.

This was causing my entire schema chain to fail validation. I've removed the xs:import from any my schemas and now its fine, but is there a problem with this .ent file?

It seems a common error in Google and I wonder if other systems are hitting it.