diff --git a/index/index.ml b/index/index.ml
index 3a5df560..2b220185 100644
--- a/index/index.ml
+++ b/index/index.ml
@@ -151,3 +151,7 @@ let term =
     $ index_name
     $ type_search
     $ favoured_prefixes)
+
+module Private = struct
+  module Suffix_tree = Suffix_tree
+end
diff --git a/index/index.mli b/index/index.mli
index fae8900e..06a369a6 100644
--- a/index/index.mli
+++ b/index/index.mli
@@ -1 +1,5 @@
 val term : (Db_store.db_format -> string -> unit) Cmdliner.Term.t
+
+module Private : sig
+  module Suffix_tree = Suffix_tree
+end
diff --git a/index/suffix_tree.ml b/index/suffix_tree.ml
index 7e96c8d6..7dadb47e 100644
--- a/index/suffix_tree.ml
+++ b/index/suffix_tree.ml
@@ -389,7 +389,7 @@ let rec export ~cache ~cache_term ~summarize ~is_root node =
       then min_child, min_child :: terminals
       else min_terminal, terminals
   in
-  assert (min_child == Seen.min_elt seen) ;
+  assert (Entry.equal min_child (Seen.min_elt seen)) ;
   assert (terminals <> []) ;
   let terminals_uid, terminals = export_terminals ~cache_term ~is_summary terminals in
   let children_uids = List.map (fun (chr, { uid; _ }) -> chr, uid) children in
diff --git a/query/priority_queue.ml b/query/priority_queue.ml
index 6c38416a..0d1a7676 100644
--- a/query/priority_queue.ml
+++ b/query/priority_queue.ml
@@ -23,8 +23,11 @@ let minimum = function
 let of_sorted_array arr = Array (0, arr)
 
 let of_automata s =
-  let elt = String_automata.minimum s in
-  All (elt, s)
+  if String_automata.size s = 0
+  then Empty
+  else (
+    let elt = String_automata.minimum s in
+    All (elt, s))
 
 let of_list lst =
   let lst = List.filter (( <> ) Empty) lst in
diff --git a/query/test/dune b/query/test/dune
index 46d89d81..59295843 100644
--- a/query/test/dune
+++ b/query/test/dune
@@ -1,3 +1,3 @@
 (test
  (name test)
- (libraries alcotest query))
+ (libraries alcotest query index))
diff --git a/query/test/test.ml b/query/test/test.ml
index 11fcf5cf..d2928537 100644
--- a/query/test/test.ml
+++ b/query/test/test.ml
@@ -1,3 +1,8 @@
 let () =
   let open Alcotest in
-  run "Query" [ "Succ", Test_succ.tests_to_seq; "Type_parser", Test_type_parser.tests ]
+  run
+    "Query"
+    [ "Succ", Test_succ.tests_to_seq
+    ; "Type_parser", Test_type_parser.tests
+    ; "Suffix_tree", Test_suffix_tree.tests
+    ]
diff --git a/query/test/test_suffix_tree.ml b/query/test/test_suffix_tree.ml
new file mode 100644
index 00000000..333e36e7
--- /dev/null
+++ b/query/test/test_suffix_tree.ml
@@ -0,0 +1,127 @@
+open Index.Private
+open Query.Private
+
+let knuth_morris_pratt str ~sub =
+  let sublen = String.length sub in
+  if sublen = 0
+  then true
+  else (
+    let len = String.length str in
+    List.init len (fun i ->
+      if i + sublen > len then false else String.sub str i sublen = sub)
+    |> List.exists Fun.id)
+
+let pkg = Db.Entry.Package.v ~name:"" ~version:""
+
+let elt cost =
+  Db.Entry.v ~cost ~name:"" ~kind:Db.Entry.Kind.Doc ~rhs:None ~doc_html:"" ~url:"" ~pkg ()
+
+let entry_of_int int =
+  Db.Entry.v
+    ~name:""
+    ~kind:Db.Entry.Kind.Module
+    ~rhs:None
+    ~doc_html:""
+    ~url:""
+    ~pkg:(Db.Entry.Package.v ~name:"" ~version:"")
+    ~cost:int
+    ()
+
+let int_of_entry entry = Db.Entry.(entry.cost)
+
+(** This module does the same thing as Succ, but its correctness is obvious
+    and its performance terrible. *)
+module Reference = struct
+  type t' = (string * int) list
+  type t = t' ref
+
+  let make () = ref []
+  let add t id entry = t := (id, entry) :: !t
+  let export t = !t
+
+  let find li request =
+    li
+    |> List.filter_map (fun (id, v) ->
+      if knuth_morris_pratt ~sub:request id then Some v else None)
+    |> List.sort_uniq Int.compare
+end
+
+module Real = struct
+  type t = Suffix_tree.t
+  type t' = Db.String_automata.t
+
+  let make () = Suffix_tree.(make (Buf.make ()))
+
+  let add t id v =
+    let entry = entry_of_int v in
+    Suffix_tree.add_suffixes t id entry
+
+  let export = Suffix_tree.export ~summarize:false
+
+  let find s req =
+    match Db.String_automata.find s req with
+    | None -> []
+    | Some automata ->
+      automata
+      |> Succ.of_automata
+      |> Succ.to_seq
+      |> List.of_seq
+      |> List.map int_of_entry
+      |> List.sort_uniq Int.compare
+end
+
+module Both = struct
+  let make () = Reference.make (), Real.make ()
+
+  let add (ref, real) id v =
+    Reference.add ref id v ;
+    Real.add real id v
+
+  let export (ref, real) = Reference.export ref, Real.export real
+  let find (ref, real) req = Reference.find ref req, Real.find real req
+end
+
+let random_string ?size () =
+  let size =
+    match size with
+    | None -> Random.int 64
+    | Some size -> size
+  in
+  String.init size (fun _ ->
+    let int = Random.int (122 - 97) + 97 in
+    Char.chr int)
+
+let random_suffix_tree size =
+  let suffixes = Both.make () in
+  for i = 1 to size do
+    let id = random_string () in
+    let v = Random.int size in
+    Printf.printf "%S -> %i\n" id v ;
+    Both.add suffixes id v
+  done ;
+  suffixes
+
+let test_find tree req =
+  let tree = Both.export tree in
+  let ref, real = Both.find tree req in
+  Alcotest.(check (list int)) "same int list" ref real
+
+let test_random_requests tree =
+  let tree = Both.export tree in
+  for i = 0 to 64 do
+    let req = random_string ~size:i () in
+    Printf.printf "req : %S\n" req ;
+    let ref, real = Both.find tree req in
+    Alcotest.(check (list int)) "same int list" ref real
+  done
+
+let test_random_size size () =
+  let tree = random_suffix_tree size in
+  test_random_requests tree
+
+let tests =
+  List.init 100 (fun i ->
+    Alcotest.test_case
+      (Printf.sprintf "Suffix_tree size %i" i)
+      `Quick
+      (test_random_size i))