diff --git a/lib/decentralised_book_index/sync/data_transformers/author_transformer.ex b/lib/decentralised_book_index/sync/data_transformers/author_transformer.ex
index 1e74db8..10acf19 100644
--- a/lib/decentralised_book_index/sync/data_transformers/author_transformer.ex
+++ b/lib/decentralised_book_index/sync/data_transformers/author_transformer.ex
@@ -20,6 +20,10 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.AuthorTransformer do
         dbi_server_id: get_in(json_body, ["attributes", "dbi_server_id"])
       }
 
-    {:ok, attrs}
+    if is_nil(attrs[:dbi_server_id]) do
+      {:ok, attrs |> Map.delete(:dbi_server_id)}
+    else
+      {:ok, attrs}
+    end
   end
 end
diff --git a/lib/decentralised_book_index/sync/data_transformers/book_transformer.ex b/lib/decentralised_book_index/sync/data_transformers/book_transformer.ex
index 5be494a..f91f54d 100644
--- a/lib/decentralised_book_index/sync/data_transformers/book_transformer.ex
+++ b/lib/decentralised_book_index/sync/data_transformers/book_transformer.ex
@@ -26,6 +26,10 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.BookTransformer do
         dbi_server_id: get_in(json_body, ["attributes", "dbi_server_id"])
       }
 
-    {:ok, attrs}
+    if is_nil(attrs[:dbi_server_id]) do
+      {:ok, attrs |> Map.delete(:dbi_server_id)}
+    else
+      {:ok, attrs}
+    end
   end
 end
diff --git a/lib/decentralised_book_index/sync/data_transformers/dbi_server_transformer.ex b/lib/decentralised_book_index/sync/data_transformers/dbi_server_transformer.ex
index 1e0728a..ab34ea1 100644
--- a/lib/decentralised_book_index/sync/data_transformers/dbi_server_transformer.ex
+++ b/lib/decentralised_book_index/sync/data_transformers/dbi_server_transformer.ex
@@ -18,6 +18,10 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.DBIServerTransformer do
         dbi_server_id: get_in(json_body, ["attributes", "dbi_server_id"])
       }
 
-    {:ok, attrs}
+    if is_nil(attrs[:dbi_server_id]) do
+      {:ok, attrs |> Map.delete(:dbi_server_id)}
+    else
+      {:ok, attrs}
+    end
   end
 end
diff --git a/lib/decentralised_book_index/sync/data_transformers/publisher_transformer.ex b/lib/decentralised_book_index/sync/data_transformers/publisher_transformer.ex
index b824873..4c44b9e 100644
--- a/lib/decentralised_book_index/sync/data_transformers/publisher_transformer.ex
+++ b/lib/decentralised_book_index/sync/data_transformers/publisher_transformer.ex
@@ -17,6 +17,10 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.PublisherTransformer do
         dbi_server_id: get_in(json_body, ["attributes", "dbi_server_id"])
       }
 
-    {:ok, attrs}
+    if is_nil(attrs[:dbi_server_id]) do
+      {:ok, attrs |> Map.delete(:dbi_server_id)}
+    else
+      {:ok, attrs}
+    end
   end
 end
diff --git a/test/decentralised_book_index/sync/data_transformers/author_transformer_test.exs b/test/decentralised_book_index/sync/data_transformers/author_transformer_test.exs
index ba37d89..013f8e4 100644
--- a/test/decentralised_book_index/sync/data_transformers/author_transformer_test.exs
+++ b/test/decentralised_book_index/sync/data_transformers/author_transformer_test.exs
@@ -81,5 +81,40 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.AuthorTransformerTest do
                updated_at: "2025-03-26T10:16:16.124581Z"
              } = author
     end
+
+    test "a json has dbi_server_id as nil, then author's map has not :dbi_server_id key" do
+      json_body = %{
+        "attributes" => %{
+          "author_alias_registry_id" => "1320a6fe-b311-45ac-bbaa-39cf29c44624",
+          "avatar_url" => "/images/avatar.png",
+          "dbi_server_id" => nil,
+          "description" => "Something",
+          "inserted_at" => "2025-03-25T19:19:15.187456Z",
+          "name" => "Author",
+          "updated_at" => "2025-03-26T10:16:16.124581Z"
+        },
+        "id" => "889a323e-d104-4b5d-b276-dad5a9b1da9d",
+        "links" => %{},
+        "meta" => %{},
+        "relationships" => %{
+          "author_alias_registry" => %{"links" => %{}, "meta" => %{}},
+          "dbi_server" => %{"links" => %{}, "meta" => %{}}
+        },
+        "type" => "author"
+      }
+
+      assert {:ok, author} = AuthorTransformer.from_json(json_body)
+
+      assert %{
+               id: "889a323e-d104-4b5d-b276-dad5a9b1da9d",
+               name: "Author",
+               description: "Something",
+               avatar_url: "/images/avatar.png",
+               author_alias_registry_id: "1320a6fe-b311-45ac-bbaa-39cf29c44624",
+               inserted_at: "2025-03-25T19:19:15.187456Z",
+               updated_at: "2025-03-26T10:16:16.124581Z"
+             } = author
+      refute Map.has_key?(author, :dbi_server_id)
+    end
   end
 end
diff --git a/test/decentralised_book_index/sync/data_transformers/book_transformer_test.exs b/test/decentralised_book_index/sync/data_transformers/book_transformer_test.exs
index 99c4999..91cdec5 100644
--- a/test/decentralised_book_index/sync/data_transformers/book_transformer_test.exs
+++ b/test/decentralised_book_index/sync/data_transformers/book_transformer_test.exs
@@ -138,5 +138,70 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.BookTransformerTest do
                dbi_server_id: "889a323e-d104-4b5d-b276-dad5a9b1da99"
              } = book
     end
+
+    test "a json has dbi_server_id as nil, then book's map has not :dbi_server_id key" do
+      json_body = %{
+        "attributes" => %{
+          "dbi_server_id" => nil,
+          "cover_image_url" => "/images/book_cover.png",
+          "description" => "A cool book.",
+          "format" => "Paper",
+          "inserted_at" => "2025-03-20T14:44:36.162986Z",
+          "language" => "English",
+          "page_count" => 1000,
+          "published" => "2025-03-05",
+          "publisher_id" => "11349865-1b7b-454a-b999-6c4059888a78",
+          "title" => "Book",
+          "updated_at" => "2025-04-01T18:14:25.754055Z"
+        },
+        "id" => "1bbe8861-9d9d-4684-bda6-b6ec238d8d08",
+        "links" => %{},
+        "meta" => %{},
+        "relationships" => %{
+          "author_roles" => %{
+            "links" => %{
+              "related" =>
+                "http://localhost:4000/api/v1/json/books/1bbe8861-9d9d-4684-bda6-b6ec238d8d08/author_roles"
+            },
+            "meta" => %{}
+          },
+          "bids" => %{
+            "links" => %{
+              "related" =>
+                "http://localhost:4000/api/v1/json/books/1bbe8861-9d9d-4684-bda6-b6ec238d8d08/bids"
+            },
+            "meta" => %{}
+          },
+          "book_editions_registry" => %{"links" => %{}, "meta" => %{}},
+          "dbi_server" => %{"links" => %{}, "meta" => %{}},
+          "publisher" => %{
+            "links" => %{
+              "related" =>
+                "http://localhost:4000/api/v1/json/books/1bbe8861-9d9d-4684-bda6-b6ec238d8d08/publisher"
+            },
+            "meta" => %{}
+          }
+        },
+        "type" => "book"
+      }
+
+      assert {:ok, book} = BookTransformer.from_json(json_body)
+
+      assert %{
+               id: "1bbe8861-9d9d-4684-bda6-b6ec238d8d08",
+               cover_image_url: "/images/book_cover.png",
+               description: "A cool book.",
+               format: "Paper",
+               inserted_at: "2025-03-20T14:44:36.162986Z",
+               language: "English",
+               page_count: 1000,
+               published: "2025-03-05",
+               publisher_id: "11349865-1b7b-454a-b999-6c4059888a78",
+               title: "Book",
+               updated_at: "2025-04-01T18:14:25.754055Z",
+             } = book
+
+      refute Map.has_key?(book, :dbi_server_id)
+    end
   end
 end
diff --git a/test/decentralised_book_index/sync/data_transformers/dbi_server_transformer_test.exs b/test/decentralised_book_index/sync/data_transformers/dbi_server_transformer_test.exs
index 7c39226..88c4eb5 100644
--- a/test/decentralised_book_index/sync/data_transformers/dbi_server_transformer_test.exs
+++ b/test/decentralised_book_index/sync/data_transformers/dbi_server_transformer_test.exs
@@ -41,6 +41,34 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.DBIServerTransformerTest
     end
 
     test "a json doesn't contains server information \"data\" attribute" do
+      json_body = %{
+        "attributes" => %{
+          "dbi_server_id" => "0c0647ec-07ef-4caa-b683-5847dbfbe5cc",
+          "inserted_at" => "2025-03-22T20:07:30.766249Z",
+          "name" => "Test",
+          "updated_at" => "2025-05-04T18:48:44.213309Z",
+          "url" => "http://localhost:4001"
+        },
+        "id" => "0c0647ec-07ef-4caa-b683-5847dbfbe5cc",
+        "links" => %{},
+        "meta" => %{},
+        "relationships" => %{"dbi_server" => %{"links" => %{}, "meta" => %{}}},
+        "type" => "dbi_server"
+      }
+
+      assert {:ok, server} = DBIServerTransformer.from_json(json_body)
+
+      assert %{
+               id: "0c0647ec-07ef-4caa-b683-5847dbfbe5cc",
+               name: "Test",
+               url: "http://localhost:4001",
+               inserted_at: "2025-03-22T20:07:30.766249Z",
+               updated_at: "2025-05-04T18:48:44.213309Z",
+               dbi_server_id: "0c0647ec-07ef-4caa-b683-5847dbfbe5cc"
+             } = server
+    end
+
+    test "a json has dbi_server_id as nil, then server's map has not :dbi_server_id key" do
       json_body = %{
         "attributes" => %{
           "inserted_at" => "2025-03-22T20:07:30.766249Z",
@@ -64,6 +92,8 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.DBIServerTransformerTest
                inserted_at: "2025-03-22T20:07:30.766249Z",
                updated_at: "2025-05-04T18:48:44.213309Z"
              } = server
+
+      refute Map.has_key?(server, :dbi_server_id)
     end
   end
 end
diff --git a/test/decentralised_book_index/sync/data_transformers/publisher_transformer_test.exs b/test/decentralised_book_index/sync/data_transformers/publisher_transformer_test.exs
index 752c1d3..adbbbc4 100644
--- a/test/decentralised_book_index/sync/data_transformers/publisher_transformer_test.exs
+++ b/test/decentralised_book_index/sync/data_transformers/publisher_transformer_test.exs
@@ -63,5 +63,32 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.PublisherTransformerTest
                dbi_server_id: "889a323e-d104-4b5d-b276-dad5a9b1da99"
              } = publisher
     end
+
+    test "a json has dbi_server_id as nil, then publisher's map has not :dbi_server_id key" do
+      json_body = %{
+        "attributes" => %{
+          "dbi_server_id" => nil,
+          "inserted_at" => "2025-03-21T09:20:48.791539Z",
+          "name" => "Publisher",
+          "updated_at" => "2025-03-21T09:20:48.791539Z"
+        },
+        "id" => "11349865-1b7b-454a-b999-6c4059888a78",
+        "links" => %{},
+        "meta" => %{},
+        "relationships" => %{"dbi_server" => %{"links" => %{}, "meta" => %{}}},
+        "type" => "publisher"
+      }
+
+      assert {:ok, publisher} = PublisherTransformer.from_json(json_body)
+
+      assert %{
+               id: "11349865-1b7b-454a-b999-6c4059888a78",
+               name: "Publisher",
+               inserted_at: "2025-03-21T09:20:48.791539Z",
+               updated_at: "2025-03-21T09:20:48.791539Z",
+             } = publisher
+
+      refute Map.has_key?(publisher, :dbi_server_id)
+    end
   end
 end