diff --git a/src/lib_stdlib_unix/lwt_utils_unix.ml b/src/lib_stdlib_unix/lwt_utils_unix.ml
index a729fd84d318e9e7e1aad4a42bf4f3094ecd2431..01bac84d833162740df2036a36eec618d8125ec0 100644
--- a/src/lib_stdlib_unix/lwt_utils_unix.ml
+++ b/src/lib_stdlib_unix/lwt_utils_unix.ml
@@ -302,6 +302,19 @@ let getpass () =
   passwd
 
 module Json = struct
+  type error += Json_decoding_error of {input : string; error : string}
+
+  let () =
+    register_error_kind
+      `Temporary
+      ~id:"json_decoding_error"
+      ~title:"Json decoding error"
+      ~description:"The input string is not a valid JSON value"
+      Data_encoding.(obj2 (req "input" string) (req "error" string))
+      (function
+        | Json_decoding_error {input; error} -> Some (input, error) | _ -> None)
+      (fun (input, error) -> Json_decoding_error {input; error})
+
   let to_root = function
     | `O ctns -> `O ctns
     | `A ctns -> `A ctns
@@ -322,7 +335,9 @@ module Json = struct
         Lwt_io.with_file ~mode:Input file (fun chan ->
             let open Lwt_result_syntax in
             let*! str = Lwt_io.read chan in
-            return (Ezjsonm.from_string str :> Data_encoding.json)))
+            match Data_encoding.Json.from_string str with
+            | Ok json -> return json
+            | Error error -> tzfail (Json_decoding_error {input = str; error})))
 end
 
 (* This module is used by [safe_cancel_on_exit] to register and unregister
diff --git a/tezt/tests/nonce_seed_revelation.ml b/tezt/tests/nonce_seed_revelation.ml
index dbef54d3dcc8d83a8e3b7c27c73e51ecda441f07..b5719a3d4cba02547738617e00313cb130de39ed 100644
--- a/tezt/tests/nonce_seed_revelation.ml
+++ b/tezt/tests/nonce_seed_revelation.ml
@@ -313,11 +313,16 @@ let test_baking_nonce_migration =
 
   Log.info "Concat old nonces contents with the new one" ;
   let new_nonces_contents = Base.read_file nonces_file in
-  let () =
-    Base.write_file
-      nonces_file
-      ~contents:(old_nonces_contents ^ new_nonces_contents)
+  let merged_nonces_contents =
+    match
+      ( Data_encoding.Json.from_string old_nonces_contents,
+        Data_encoding.Json.from_string new_nonces_contents )
+    with
+    | Ok (`A old), Ok (`A new_) ->
+        Data_encoding.Json.to_string (`A (old @ new_))
+    | _ -> Test.fail "Incorrectly formatted nonces files"
   in
+  let () = Base.write_file nonces_file ~contents:merged_nonces_contents in
 
   Log.info
     "Restart the baker and wait for ignore failed nonce migration event then \