history command that I can't get to get properly typed

history-cmd.clj

(t/ann ^:no-check history-cmd [t2/Game -> String])
(defn history-cmd
  "The history command."
  [game]
  (str "*** START HISTORY ***"
       (w/walk #(str "\n> " (:command %) "\n\n" (:response %) "\n")
               #(apply str %)
               (if (< (count (:turn-history game)) 4)
                 (:turn-history game)
                 (subvec (:turn-history game) (- (count (:turn-history game)) 4))))
       "*** END HISTORY ***"))

;; Relevant custom types
(t/defalias Player
  "The player."
  (t/HMap :mandatory {:name String}))

(t/defalias Turn
  "A single player turn including command and response."
  (t/HMap :mandatory {:command t/Symbol
                      :response String}
          :optional {:invalid t/Bool}))

(t/defalias Room
  "A room in an area of the world."
  (t/HMap :mandatory {:name String}))

(t/defalias Biome
  "This is just an alias for String, to hold a biome name."
  String)

(t/defalias Area
  "An area of the world."
  (t/HMap :mandatory {:name String
                      :type Biome}
          :optional {:rooms (t/Vec (t/Option Room))
                     :trees t/Num}))

(t/defalias World
  "The world."
  (t/HMap :mandatory {:areas (t/Vec Area)}))

(t/defalias Game
  "The game."
  (t/HMap :mandatory {:player Player
                      :world World
                      :turn-history (t/Vec (t/Option Turn))}
          :optional {:last-turn Turn}))

lein typed output

Initializing core.typed ...
Building core.typed base environments ...
Finished building base environments
"Elapsed time: 6687.601 msecs"
core.typed initialized.
Start collecting zork-fortress.core
Start collecting zork-fortress.cmds
Start collecting zork-fortress.types
Finished collecting zork-fortress.types
Finished collecting zork-fortress.cmds
Finished collecting zork-fortress.core
Collected 3 namespaces in 1159.468 msecs
Not checking clojure.core.typed (tagged :collect-only in ns metadata)
Start checking zork-fortress.types
Checked zork-fortress.types in 760.991 msecs
Not checking clojure.walk (tagged :collect-only in ns metadata)
Start checking zork-fortress.cmds
Checked zork-fortress.cmds in 1127.613 msecs
Start checking zork-fortress.core
Checked zork-fortress.core in 291.88 msecs
WARNING: Type Checker: Definition missing: clojure.walk/walk 
Hint: Use :no-check metadata with ann if this is an unchecked var
Checked 5 namespaces (approx. 2553 lines) in 3379.838 msecs
Type Error (zork_fortress/cmds.clj:17:9) Unannotated var clojure.walk/walk
Hint: Add the annotation for clojure.walk/walk via check-ns or cf
in: w/walk


Type Error (zork_fortress/cmds.clj:18:17) Bad arguments to apply: 

Target: 	(t/IFn [t/Any * -> java.lang.String])

Arguments:	t/Any
in: (apply str p1__520#)


Type Checker: Found 2 errors
Found errors

I get the same error after replacing #(apply str %) with (fn [a] {:pre [(or (nil? a) (coll? a))]} (apply str a)). The only change I see in the output is in: (apply str a) instead of in: (apply str p1__520#).

Additionaly, I would never have figured out the issue was with apply expecting a sequence. How should I have read the output to figure that out? I feel like I'm just missing something.

This was the solution:

(t/ann clojure.walk/walk [[t/Any -> t/Any] [t/Any -> t/Any] t/Any -> t/Any])
(t/ann history-cmd [t2/Game -> String])
(defn history-cmd
  "The history command."
  [game]
  (str "*** START HISTORY ***"
       (w/walk #(str "\n> " (:command %) "\n\n" (:response %) "\n")
               (fn [a] {:pre [((t/pred (t/U nil (t/Coll t/Any))) a)]} (apply str a))
               (if (< (count (:turn-history game)) 4)
                 (:turn-history game)
                 (subvec (:turn-history game) (- (count (:turn-history game)) 4))))
       "*** END HISTORY ***"))