diff --git a/src/register.eml.html b/src/register.eml.html
index ff1fbb4..0cc46f0 100644
--- a/src/register.eml.html
+++ b/src/register.eml.html
@@ -5,7 +5,7 @@ let f ?email ?nick ?password request =
 %     begin match User.register ~email ~nick ~password with
 %     | Error e ->
 Error: <%s e %>
-%     | Ok _ ->
+%     | Ok _ (* TODO: do not match on _ *) ->
 User created !
 %     end;
 %   | _ ->
diff --git a/src/user.ml b/src/user.ml
index 056750e..1ce3507 100644
--- a/src/user.ml
+++ b/src/user.ml
@@ -48,14 +48,24 @@ let register ~email ~nick ~password =
   if not valid then
     Error "Something is wrong"
   else
-    (* TODO: add check uniqueness of id *)
     let open Sqlite3_utils in
-    let res =
+    let unique =
       Db.with_db (fun db ->
-          exec_raw_args db "INSERT INTO user VALUES (?, ?, ?);"
-            [| Data.TEXT nick; Data.TEXT password; Data.TEXT email |]
+          exec_raw_args db
+            "SELECT EXISTS(SELECT 1 FROM user WHERE nick=? OR email=?);"
+            [| Data.TEXT nick; Data.TEXT email |]
             ~f:Cursor.to_list )
     in
-    match res with
-    | Ok res -> Ok res
+    match unique with
+    | Ok [ [| Data.INT 0L |] ] -> (
+      let res =
+        Db.with_db (fun db ->
+            exec_raw_args db "INSERT INTO user VALUES (?, ?, ?);"
+              [| Data.TEXT nick; Data.TEXT password; Data.TEXT email |]
+              ~f:Cursor.to_list )
+      in
+      match res with
+      | Ok res -> Ok res
+      | Error e -> Error (Format.sprintf "db error: %s" (Rc.to_string e)) )
+    | Ok _ -> Error "nick or email already exists"
     | Error e -> Error (Format.sprintf "db error: %s" (Rc.to_string e))