why would this fail in core.logic?

[Jim - FooBar();]

caution! logic noob question following...why does the following fail? 
can anyone help with this? the stack trace is not very helpful!

(defn bishop-moves [x y]
  (let [xmax 8 ymax 8]
    (run* [q]
      (fresh [a b]
      (< a xmax)
      (< b ymax)
      (= (Math/abs (- x a))
           (Math/abs (- y b)))
            (== q [a b])))))

thanks in advance,

Jim

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