A schema import that specifies a prefix has a zero-length namespace name, which is not allowed. For example, the following schema import is invalid:

import schema namespace prod = ""
              at "http://datypic.com/prod.xsd";

You can, however, import a schema with no target namespace if you make "no namespace" the default, as in:

import schema default element namespace ""
              at "http://datypic.com/prod.xsd";