While working on a project I found a somewhat strange behaviour. I reworked the problem in some simple predicate. Let's take this predicate foobad.

(defn foobad 
  [?bar]
  (l/project [?bar]
    (l/fresh [?var]
      (l/conde 
        [(l/== true (instance? js/Array ?bar))
          (membero ?var (seq ?bar))]))))

When running (l/run* [?f] (foobad 0)) it produces this error:

#<Error: No protocol method ISeqable.-seq defined for type number: 0>

Because 0 is not an instance of Array it should not be entering that conde clause where it tries (seq 0).

With this workaround we get a normal behaviour:

(defn foo 
  [?bar]
  (l/project [?bar]
    (l/fresh [?var]
      (l/conde 
        [(l/== true (instance? js/Array ?bar)) 
         (fresh [?s]
          (l/== ?s (seq ?bar))
          (membero ?var ?s))]))))

Running (l/run* [?f] (foo 0)) gives us (), which is what we expect.
While 0 is clearly not an instance of Array that conde clause is still executed resulting in previous error.