XPaths are queries. They don't validate. Your request to help you turn off validation cannot be satisfied.

And then there's the fact that the concept of defining a namespace makes no sense. Every single uri is already available to use as a namespace. Nothing needs to be defined. This makes your error message suspicious. Please provide the actual error message.

You'd also do well to provide a minimal example of the problem. It's hard and annoying to fix your error without seeing it.