diff --git a/content/reference/reducers.adoc b/content/reference/reducers.adoc
index 985e6158..22cac16e 100644
--- a/content/reference/reducers.adoc
+++ b/content/reference/reducers.adoc
@@ -64,10 +64,10 @@ Use https://clojure.github.io/clojure/clojure.core-api.html#clojure.core/into[in
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 (into [] (r/filter even? (r/map inc (range 100000))))
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-Or https://clojure.github.io/clojure/clojure.core-api.html#clojure.core.reducers/foldcat[r/foldcat]:
+Or https://clojure.github.io/clojure/clojure.core-api.html#clojure.core.reducers/foldcat[r/foldcat] to produce a foldable collection. Note the use of `vec` — seqs like those returned by `range` do not support parallel folding, so they must be converted to a vector first:
 [source,clojure]
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-(r/foldcat (r/filter even? (r/map inc (range 100000))))
+(r/foldcat (r/filter even? (r/map inc (vec (range 100000)))))
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 Specify a reduce function and a combine function with fold:
 [source,clojure]