Jaynes (1973 and 2003) provides a very insightful discussion of this riddle and also argues that it may be resolved by relying on invariances of the problem under certain transformations. But the general message for now is that the principle of indifference does not lead to a unique choice of priors.  The point is not that ignorance concerning a parameter is hard to express in a probability distribution over those values. It is rather that in some cases, we do not even know what parameters to use to express our ignorance over.