While this is not impossible it is very rare. I would suggest you read the following link, and perhaps print this off and provide it to you veterinarian for assistance with a definitive diagnosis. This may also help your vet to understand the etiology and treatment of Fanconi Syndrome in your pet:

https://www.basenji.org/ClubDocs/fanconiprotocol2003.pdf

Good luck, and I hope your weim finds some comfort.