diff --git a/lib/membership/email.ex b/lib/membership/email.ex
index 47651f5..730ccd7 100644
--- a/lib/membership/email.ex
+++ b/lib/membership/email.ex
@@ -8,10 +8,11 @@ defmodule Mv.Membership.Email do
   addresses according to a standard regex pattern.
 
   ## Validation Rules
-  - Minimum length: 5 characters
+  - **Optional**: `nil` and empty strings are allowed (custom fields are optional)
+  - Minimum length: 5 characters (for non-empty values)
   - Maximum length: 254 characters (RFC 5321 maximum)
   - Pattern: Standard email format (username@domain.tld)
-  - Automatic trimming of leading/trailing whitespace
+  - Automatic trimming of leading/trailing whitespace (empty strings become `nil`)
 
   ## Usage
   This type is used in the CustomFieldValue union type for custom fields with
@@ -46,11 +47,18 @@ defmodule Mv.Membership.Email do
       max_length: @max_length
     ]
 
+  @impl true
+  def cast_input(nil, _), do: {:ok, nil}
+
   @impl true
   def cast_input(value, _) when is_binary(value) do
     value = String.trim(value)
 
     cond do
+      # Empty string after trim becomes nil (optional field)
+      value == "" ->
+        {:ok, nil}
+
       String.length(value) < @min_length ->
         :error
 
diff --git a/test/membership/custom_field_validation_test.exs b/test/membership/custom_field_validation_test.exs
index d8a5bd9..a5c1f2d 100644
--- a/test/membership/custom_field_validation_test.exs
+++ b/test/membership/custom_field_validation_test.exs
@@ -1,7 +1,7 @@
 defmodule Mv.Membership.CustomFieldValidationTest do
   @moduledoc """
   Tests for CustomField validation constraints.
-  
+
   Tests cover:
   - Name length validation (max 100 characters)
   - Name trimming
@@ -203,4 +203,3 @@ defmodule Mv.Membership.CustomFieldValidationTest do
     end
   end
 end
-
diff --git a/test/membership/custom_field_value_validation_test.exs b/test/membership/custom_field_value_validation_test.exs
index ce5b5c6..dd3438a 100644
--- a/test/membership/custom_field_value_validation_test.exs
+++ b/test/membership/custom_field_value_validation_test.exs
@@ -1,7 +1,7 @@
 defmodule Mv.Membership.CustomFieldValueValidationTest do
   @moduledoc """
   Tests for CustomFieldValue validation constraints.
-  
+
   Tests cover:
   - String value length validation (max 10,000 characters)
   - String value trimming
@@ -184,6 +184,36 @@ defmodule Mv.Membership.CustomFieldValueValidationTest do
   end
 
   describe "email value validation" do
+    test "accepts nil value (optional field)", %{member: member, email_field: email_field} do
+      assert {:ok, custom_field_value} =
+               CustomFieldValue
+               |> Ash.Changeset.for_create(:create, %{
+                 member_id: member.id,
+                 custom_field_id: email_field.id,
+                 value: %{"_union_type" => "email", "_union_value" => nil}
+               })
+               |> Ash.create()
+
+      assert custom_field_value.value.value == nil
+    end
+
+    test "accepts empty string (becomes nil after trim)", %{
+      member: member,
+      email_field: email_field
+    } do
+      assert {:ok, custom_field_value} =
+               CustomFieldValue
+               |> Ash.Changeset.for_create(:create, %{
+                 member_id: member.id,
+                 custom_field_id: email_field.id,
+                 value: %{"_union_type" => "email", "_union_value" => ""}
+               })
+               |> Ash.create()
+
+      # Empty string after trim should become nil
+      assert custom_field_value.value.value == nil
+    end
+
     test "accepts valid email", %{member: member, email_field: email_field} do
       assert {:ok, custom_field_value} =
                CustomFieldValue
@@ -273,4 +303,3 @@ defmodule Mv.Membership.CustomFieldValueValidationTest do
     end
   end
 end
-