diff --git a/Gemfile b/Gemfile
index 71e4bf3..50911f3 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,3 +5,4 @@ git_source(:github) { |repo_name| "https://github.com/#{repo_name}" }
 
 # Specify your gem's dependencies in valkyrie-sequel.gemspec
 gemspec
+gem "valkyrie", branch: "837-persisters-should-error", github: "samvera/valkyrie"
diff --git a/lib/valkyrie/sequel/persister.rb b/lib/valkyrie/sequel/persister.rb
index 5149f96..6b8ab50 100644
--- a/lib/valkyrie/sequel/persister.rb
+++ b/lib/valkyrie/sequel/persister.rb
@@ -124,8 +124,12 @@ def wipe!
     def create_or_update(resource:, attributes:)
       attributes[:updated_at] = Time.now.utc
       attributes[:created_at] ||= Time.now.utc
-      return create(resource: resource, attributes: attributes) unless resource.persisted? && !exists?(id: attributes[:id])
-      update(resource: resource, attributes: attributes)
+      if resource.persisted?
+        raise Valkyrie::Persistence::ObjectNotFoundError, "The object #{resource.id} is previously persisted but not found at save time." unless exists?(id: attributes[:id])
+        update(resource: resource, attributes: attributes)
+      else
+        create(resource: resource, attributes: attributes) unless resource.persisted? && !exists?(id: attributes[:id])
+      end
     end
 
     def create(resource:, attributes:)
@@ -146,7 +150,7 @@ def update(resource:, attributes:)
     end
 
     def exists?(id:)
-      resources.select(1).first(id: id).nil?
+      !resources.select(1).first(id: id).nil?
     end
   end
 end