diff --git a/CHANGELOG.md b/CHANGELOG.md
index ac8acd7..5e2e9fd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 ### Fixed
+- Fix use of `User#expires_at` in `SpecHelpers#stub_auth_for` (#82)
 
 ### Removed
 
diff --git a/lib/rpi_auth/spec_helpers.rb b/lib/rpi_auth/spec_helpers.rb
index f371d26..4c6fc8d 100644
--- a/lib/rpi_auth/spec_helpers.rb
+++ b/lib/rpi_auth/spec_helpers.rb
@@ -6,7 +6,8 @@ module SpecHelpers
     # model has an `:user_id` method which returns the users ID, but this can
     # be changed by setting the `id_param` option.
     def stub_auth_for(user:, id_param: :user_id) # rubocop:disable Metrics/AbcSize
-      expires_in = user.respond_to?(:expires_at) ? user.expires_at.to_i - Time.zone.now.to_i : 3600
+      expires_at = user.respond_to?(:expires_at) && user.expires_at
+      expires_in = expires_at.present? ? expires_at.to_i - Time.zone.now.to_i : 3600
       token = user.respond_to?(:access_token) ? user.access_token : SecureRandom.hex(16)
       refresh_token = user.respond_to?(:refresh_token) ? user.refresh_token : SecureRandom.hex(16)