diff --git a/app/models/organization.rb b/app/models/organization.rb
index fc5c74024..09d1ec53a 100644
--- a/app/models/organization.rb
+++ b/app/models/organization.rb
@@ -134,6 +134,12 @@ def self.instance
     first!
   end
 
+  def icalendar_auth_token
+    super
+  rescue ActiveRecord::Encryption::Errors::Decryption
+    nil
+  end
+
   def features
     self[:features].map(&:to_sym) & FEATURES
   end
diff --git a/test/controllers/activity_participations_calendar_controller_test.rb b/test/controllers/activity_participations_calendar_controller_test.rb
index 80e25cae0..068c5703e 100644
--- a/test/controllers/activity_participations_calendar_controller_test.rb
+++ b/test/controllers/activity_participations_calendar_controller_test.rb
@@ -15,6 +15,12 @@ def request(auth_token: nil)
     assert_response :unauthorized
   end
 
+  test "with an undecryptable auth token" do
+    corrupt_icalendar_auth_token!
+    request(auth_token: "1234abcd")
+    assert_response :unauthorized
+  end
+
   test "with a wrong auth token" do
     request(auth_token: "wrong")
     assert_response :unauthorized
diff --git a/test/controllers/activity_participations_controller_test.rb b/test/controllers/activity_participations_controller_test.rb
new file mode 100644
index 000000000..aeda3c153
--- /dev/null
+++ b/test/controllers/activity_participations_controller_test.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class ActivityParticipationsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    host! "admin.acme.test"
+  end
+
+  def login(admin)
+    session = Session.create!(
+      admin_email: admin.email,
+      remote_addr: "127.0.0.1",
+      user_agent: "Test Browser")
+    get "/sessions/#{session.generate_token_for(:redeem)}"
+  end
+
+  test "index shows calendar subscribe link when icalendar_auth_token is present" do
+    travel_to "2024-01-01"
+    login admins(:super)
+
+    get activity_participations_path
+
+    assert_response :success
+    assert_select "#calendar_sidebar_section"
+  end
+
+  test "index succeeds when icalendar_auth_token cannot be decrypted" do
+    travel_to "2024-01-01"
+    login admins(:super)
+    corrupt_icalendar_auth_token!
+
+    get activity_participations_path
+
+    assert_response :success
+    assert_select "#calendar_sidebar_section", false
+  end
+end
diff --git a/test/models/organization_test.rb b/test/models/organization_test.rb
index 2c19bc448..4cfb286be 100644
--- a/test/models/organization_test.rb
+++ b/test/models/organization_test.rb
@@ -392,4 +392,12 @@ class OrganizationTest < ActiveSupport::TestCase
     assert Current.org.encrypted_attribute?(:api_token)
     assert Current.org.encrypted_attribute?(:icalendar_auth_token)
   end
+
+  test "icalendar_auth_token presence is false when ciphertext cannot be decrypted" do
+    corrupt_icalendar_auth_token!
+    organization = Organization.uncached { Organization.find(Current.org.id) }
+
+    assert_nil organization.icalendar_auth_token
+    assert_not organization.icalendar_auth_token?
+  end
 end
diff --git a/test/support/organizations_helper.rb b/test/support/organizations_helper.rb
index 8d9ea3cfd..5f7af9607 100644
--- a/test/support/organizations_helper.rb
+++ b/test/support/organizations_helper.rb
@@ -5,6 +5,18 @@ def org(columns = {})
     Current.org.update_columns(columns)
   end
 
+  def corrupt_icalendar_auth_token!(organization = Current.org)
+    ciphertext = JSON.parse(organization.ciphertext_for(:icalendar_auth_token))
+    ciphertext["p"] = ciphertext.fetch("p").tr("A-Za-z0-9", "B-ZAb-z0-91")
+    # Raw SQL: update_column would re-encrypt this payload as plaintext.
+    Organization.connection.update(
+      Organization.sanitize_sql_array([
+        "UPDATE organizations SET icalendar_auth_token = ? WHERE id = ?",
+        ciphertext.to_json,
+        organization.id
+      ]))
+  end
+
   def german_org(columns = {})
     attrs = {
       languages: [ "de" ],