diff --git a/src/node/shell/distributed_db.ml b/src/node/shell/distributed_db.ml
index 6b214323a..a3a073249 100644
--- a/src/node/shell/distributed_db.ml
+++ b/src/node/shell/distributed_db.ml
@@ -875,12 +875,14 @@ module type DISTRIBUTED_DB = sig
   type param
   val known: t -> key -> bool Lwt.t
   type error += Missing_data of key
+  type error += Canceled of key
   val read: t -> key -> value tzresult Lwt.t
   val read_opt: t -> key -> value option Lwt.t
   val read_exn: t -> key -> value Lwt.t
   val watch: t -> (key * value) Lwt_stream.t * Watcher.stopper
   val prefetch: t -> ?peer:P2p.Peer_id.t -> key -> param -> unit
-  val fetch: t -> ?peer:P2p.Peer_id.t -> key -> param -> value Lwt.t
+  val fetch:
+    t -> ?peer:P2p.Peer_id.t -> key -> param -> value tzresult Lwt.t
   val clear_or_cancel: t -> key -> unit
 end
 
@@ -896,6 +898,7 @@ module Make
   type param = Table.param
   let known t k = Table.known (Kind.proj t) k
   type error += Missing_data = Table.Missing_data
+  type error += Canceled = Table.Canceled
   let read t k = Table.read (Kind.proj t) k
   let read_opt t k = Table.read_opt (Kind.proj t) k
   let read_exn t k = Table.read_exn (Kind.proj t) k
diff --git a/src/node/shell/distributed_db.mli b/src/node/shell/distributed_db.mli
index 7dec7d299..b90e16847 100644
--- a/src/node/shell/distributed_db.mli
+++ b/src/node/shell/distributed_db.mli
@@ -78,12 +78,14 @@ module type DISTRIBUTED_DB = sig
   type param
   val known: t -> key -> bool Lwt.t
   type error += Missing_data of key
+  type error += Canceled of key
   val read: t -> key -> value tzresult Lwt.t
   val read_opt: t -> key -> value option Lwt.t
   val read_exn: t -> key -> value Lwt.t
   val watch: t -> (key * value) Lwt_stream.t * Watcher.stopper
   val prefetch: t -> ?peer:P2p.Peer_id.t -> key -> param -> unit
-  val fetch: t -> ?peer:P2p.Peer_id.t -> key -> param -> value Lwt.t
+  val fetch:
+    t -> ?peer:P2p.Peer_id.t -> key -> param -> value tzresult Lwt.t
   val clear_or_cancel: t -> key -> unit
 end
 
diff --git a/src/node/shell/distributed_db_functors.ml b/src/node/shell/distributed_db_functors.ml
index 60cda7335..0a50d3a5a 100644
--- a/src/node/shell/distributed_db_functors.ml
+++ b/src/node/shell/distributed_db_functors.ml
@@ -19,12 +19,14 @@ module type DISTRIBUTED_DB = sig
   val known: t -> key -> bool Lwt.t
 
   type error += Missing_data of key
+  type error += Canceled of key
   val read: t -> key -> value tzresult Lwt.t
   val read_opt: t -> key -> value option Lwt.t
   val read_exn: t -> key -> value Lwt.t
 
   val prefetch: t -> ?peer:P2p.Peer_id.t -> key -> param -> unit
-  val fetch: t -> ?peer:P2p.Peer_id.t -> key -> param -> value Lwt.t
+  val fetch:
+    t -> ?peer:P2p.Peer_id.t -> key -> param -> value tzresult Lwt.t
 
   val clear_or_cancel: t -> key -> unit
   val inject: t -> key -> value -> bool Lwt.t
@@ -108,7 +110,7 @@ end = struct
   }
 
   and status =
-    | Pending of { wakener : value Lwt.u ;
+    | Pending of { wakener : value tzresult Lwt.u ;
                    mutable waiters : int ;
                    param : param }
     | Found of value
@@ -132,6 +134,7 @@ end = struct
     | Pending _ -> Lwt.fail Not_found
 
   type error += Missing_data of key
+  type error += Canceled of key
 
   let () =
     Error_monad.register_error_kind `Permanent
@@ -171,7 +174,7 @@ end = struct
     match Memory_table.find s.memory k with
     | exception Not_found -> begin
         Disk_table.read_opt s.disk k >>= function
-        | Some v -> Lwt.return v
+        | Some v -> return v
         | None ->
             match Memory_table.find s.memory k with
             | exception Not_found -> begin
@@ -185,13 +188,13 @@ end = struct
                 Scheduler.request s.scheduler peer k ;
                 data.waiters <- data.waiters + 1 ;
                 wrap s k (Lwt.waiter_of_wakener data.wakener)
-            | Found v -> Lwt.return v
+            | Found v -> return v
       end
     | Pending data ->
         Scheduler.request s.scheduler peer k ;
         data.waiters <- data.waiters + 1 ;
         wrap s k (Lwt.waiter_of_wakener data.wakener)
-    | Found v -> Lwt.return v
+    | Found v -> return v
 
   let prefetch s ?peer k param = Lwt.ignore_result (fetch s ?peer k param)
 
@@ -214,7 +217,7 @@ end = struct
         | Some v ->
             Scheduler.notify s.scheduler p k ;
             Memory_table.replace s.memory k (Found v) ;
-            Lwt.wakeup w v ;
+            Lwt.wakeup_later w (Ok v) ;
             iter_option s.global_input
               ~f:(fun input -> Watcher.notify input (k, v)) ;
             Watcher.notify s.input (k, v) ;
@@ -244,7 +247,7 @@ end = struct
     | Pending { wakener = w ; _ } ->
         Scheduler.notify_cancelation s.scheduler k ;
         Memory_table.remove s.memory k ;
-        Lwt.wakeup_later_exn w Lwt.Canceled
+        Lwt.wakeup_later w (Error [Canceled k])
     | Found _ -> Memory_table.remove s.memory k
 
   let watch s = Watcher.create_stream s.input
diff --git a/src/node/shell/distributed_db_functors.mli b/src/node/shell/distributed_db_functors.mli
index 756e5a20e..9babd8762 100644
--- a/src/node/shell/distributed_db_functors.mli
+++ b/src/node/shell/distributed_db_functors.mli
@@ -17,12 +17,14 @@ module type DISTRIBUTED_DB = sig
   val known: t -> key -> bool Lwt.t
 
   type error += Missing_data of key
+  type error += Canceled of key
   val read: t -> key -> value tzresult Lwt.t
   val read_opt: t -> key -> value option Lwt.t
   val read_exn: t -> key -> value Lwt.t
 
   val prefetch: t -> ?peer:P2p.Peer_id.t -> key -> param -> unit
-  val fetch: t -> ?peer:P2p.Peer_id.t -> key -> param -> value Lwt.t
+  val fetch:
+    t -> ?peer:P2p.Peer_id.t -> key -> param -> value tzresult Lwt.t
 
   val clear_or_cancel: t -> key -> unit
   val inject: t -> key -> value -> bool Lwt.t
diff --git a/src/node/shell/validator.ml b/src/node/shell/validator.ml
index 187cf7c30..78e61eb2d 100644
--- a/src/node/shell/validator.ml
+++ b/src/node/shell/validator.ml
@@ -66,7 +66,7 @@ let bootstrapped v = v.bootstrapped
 let fetch_protocol v hash =
   lwt_log_notice "Fetching protocol %a"
     Protocol_hash.pp_short hash >>= fun () ->
-  Distributed_db.Protocol.fetch v.worker.db hash () >>= fun protocol ->
+  Distributed_db.Protocol.fetch v.worker.db hash () >>=? fun protocol ->
   Updater.compile hash protocol >>= fun valid ->
   if valid then begin
     lwt_log_notice "Successfully compiled protocol %a"
@@ -253,7 +253,7 @@ let apply_block net_state db
   lwt_log_info "validation of %a: looking for dependencies..."
     Block_hash.pp_short hash >>= fun () ->
   Distributed_db.Operations.fetch
-    db (hash, 0) block.shell.operations_hash >>= fun operations ->
+    db (hash, 0) block.shell.operations_hash >>=? fun operations ->
   fail_unless (block.shell.validation_passes <= 1)
     (* TODO constant to be exported from the protocol... *)
     (failure "unexpected error (TO BE REMOVED)") >>=? fun () ->
@@ -399,7 +399,7 @@ module Context_db = struct
       assert (not (Block_hash.Table.mem tbl hash));
       let waiter, wakener = Lwt.wait () in
       let data =
-        Distributed_db.Block_header.fetch net_db hash () >>= return in
+        Distributed_db.Block_header.fetch net_db hash () in
       match Lwt.state data with
       | Lwt.Return data ->
           let state = `Inited data in
@@ -733,13 +733,13 @@ let rec create_validator ?parent worker ?max_child_ttl state db net =
           Block_hash.equal (State.Net.genesis child.net).block genesis in
     begin
       match max_child_ttl with
-      | None -> Lwt.return expiration
+      | None -> return expiration
       | Some ttl ->
-          Distributed_db.Block_header.fetch net_db genesis () >>= fun genesis ->
-          Lwt.return
+          Distributed_db.Block_header.fetch net_db genesis () >>=? fun genesis ->
+          return
             (Time.min expiration
                (Time.add genesis.shell.timestamp (Int64.of_int ttl)))
-    end >>= fun local_expiration ->
+    end >>=? fun local_expiration ->
     let expired = Time.(local_expiration <= current_time) in
     if expired && activated then
       deactivate_child () >>= return