diff --git a/lib/sequent/core/aggregate_root.rb b/lib/sequent/core/aggregate_root.rb
index 67f031a2..e6c1de10 100644
--- a/lib/sequent/core/aggregate_root.rb
+++ b/lib/sequent/core/aggregate_root.rb
@@ -98,6 +98,26 @@ def to_s
         "#{self.class.name}: #{@id}"
       end
 
+      # Some aggregates represent a unique external entity (e.g. a
+      # user's email address or login name) and this uniqueness needs
+      # to be enforced. For each unique key the returned object should
+      # have an entry where the key of the entry describes the scope
+      # of the constraint (e.g. `user_email` or `login_name`) and the
+      # value represents the unique value. Values can be any JSON
+      # value (string, object, array, etc). Note that uniqueness is
+      # enforced across all aggregate types if the same scope is used.
+      #
+      # An `AggregateKeyNotUniqueError` is raised if a unique
+      # constrained is violated when committing the events to the
+      # database.
+      #
+      # Sample return value:
+      #
+      # ```
+      # {
+      #   user_email: { email: 'bob@example.com' }
+      # }
+      # ```
       def unique_keys
         {}
       end
diff --git a/lib/sequent/core/event_store.rb b/lib/sequent/core/event_store.rb
index 4af13063..929b7a15 100644
--- a/lib/sequent/core/event_store.rb
+++ b/lib/sequent/core/event_store.rb
@@ -9,6 +9,9 @@
 
 module Sequent
   module Core
+    class AggregateKeyNotUniqueError < RuntimeError
+    end
+
     class EventStore
       include Helpers::PgsqlHelpers
       include SnapshotStore
@@ -18,9 +21,6 @@ class EventStore
       class OptimisticLockingError < RuntimeError
       end
 
-      class AggregateKeyNotUniqueError < RuntimeError
-      end
-
       class DeserializeEventError < RuntimeError
         attr_reader :event_hash
 
diff --git a/spec/lib/sequent/core/aggregate_repository_spec.rb b/spec/lib/sequent/core/aggregate_repository_spec.rb
index 6439960f..6e6ae85d 100644
--- a/spec/lib/sequent/core/aggregate_repository_spec.rb
+++ b/spec/lib/sequent/core/aggregate_repository_spec.rb
@@ -509,7 +509,7 @@ def unique_keys
         Sequent.aggregate_repository.add_aggregate(dummy2)
 
         expect { Sequent.aggregate_repository.commit(DummyCommand.new) }
-          .to raise_error Sequent::Core::EventStore::AggregateKeyNotUniqueError
+          .to raise_error Sequent::Core::AggregateKeyNotUniqueError
       end
     end
   end