diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0033e57..ad86f59 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased] - 
 
+### Added
+- Add `Bill::User#billing_details` abstract method
+
 ### Changed
 - Upgrade GitHub actions
 - Limit description lengths to mitigate potential DoS
diff --git a/spec/support/app/src/models/user.cr b/spec/support/app/src/models/user.cr
index 2785f13..ad15626 100644
--- a/spec/support/app/src/models/user.cr
+++ b/spec/support/app/src/models/user.cr
@@ -19,7 +19,7 @@ class User < BaseModel
     "User ##{id}"
   end
 
-  def billing_details
+  def billing_details : String
     <<-TEXT
     #{full_name}
     <#{email}>
diff --git a/src/bill/models/user.cr b/src/bill/models/user.cr
index 3159bcd..828cc1d 100644
--- a/src/bill/models/user.cr
+++ b/src/bill/models/user.cr
@@ -1,4 +1,6 @@
 module Bill::User
+  abstract def billing_details : String
+
   macro included
   end
 end