Instrumenting generators trouble

core.clj

(ns test.core
  (:require [clojure.spec :as s]
            [clojure.spec.test :as stest]
            [clojure.test.check.generators :as gen]))

(s/def ::file-count nat-int?)
(s/def ::operations (s/map-of keyword? pos-int?))
(s/def ::time-total (s/and double? (complement neg?)))

(s/def ::summary-res
  (s/keys :req-un [::file-count
                   ::operations
                   ::time-total]))

(s/fdef combine-summaries
  :args (s/cat :xs (s/coll-of ::summary-res :min-count 1))
  :ret ::summary-res)

(defn combine-summaries
  [summaries]
  (letfn [(deep-merge [a b]
            (merge-with (fn [x y]
                          (if (map? y)
                            (deep-merge x y)
                            (+ x y)))
                        a b))]
    (reduce deep-merge summaries)))

repl-session.clj

test.core> (->> (stest/check
                  `combine-summaries
                  {:clojure.spec.test.check/opts {:num-tests 1000}})
                first
                stest/abbrev-result)
{:spec (fspec
        :args (cat :xs (coll-of :test.core/summary-res :min-count 1))
        :ret :test.core/summary-res
        :fn nil),
 :sym test.core/combine-summaries,
 :failure #object[java.lang.ArithmeticException "0x1c497200" "java.lang.ArithmeticException: integer overflow"]}
test.core> (stest/instrument (stest/enumerate-namespace 'test.core)
                             {:gen {::file-count (s/gen (s/int-in 0 1000))
                                    ::operations (s/gen (s/map-of keyword? (s/int-in 1 1000)))
                                    ::time-total (s/gen (s/double-in 0.0 1000.0))}})
[test.core/combine-summaries]
test.core> (->> (stest/check
                  `combine-summaries
                  {:clojure.spec.test.check/opts {:num-tests 1000}})
                first
                stest/abbrev-result)
{:spec (fspec
         :args (cat :xs (coll-of :test.core/summary-res :min-count 1))
         :ret :test.core/summary-res
         :fn nil),
 :sym test.core/combine-summaries,
 :failure #object[java.lang.ArithmeticException "0x3894d917" "java.lang.ArithmeticException: integer overflow"]}
test.core>