diff --git a/activerecord/lib/active_record/signed_id.rb b/activerecord/lib/active_record/signed_id.rb
index a553e3d2f47b7c2420d9d4ddb58ab29b05073403..abd7ddf37e9ab8e71b45f235ab28925d33c0ffcf 100644
--- a/activerecord/lib/active_record/signed_id.rb
+++ b/activerecord/lib/active_record/signed_id.rb
@@ -40,8 +40,10 @@ module ClassMethods
       #   travel_back
       #   User.find_signed signed_id, purpose: :password_reset # => User.first
       def find_signed(signed_id, purpose: nil)
+        raise UnknownPrimaryKey.new(@klass) if primary_key.nil?
+
         if id = signed_id_verifier.verified(signed_id, purpose: combine_signed_id_purposes(purpose))
-          find_by id: id
+          find_by primary_key => id
         end
       end
 
diff --git a/activerecord/test/cases/signed_id_test.rb b/activerecord/test/cases/signed_id_test.rb
index 0b963220c058494a605be4241f2aa52830f14757..9933401ebe83d0834100ade79399c34c32b9d758 100644
--- a/activerecord/test/cases/signed_id_test.rb
+++ b/activerecord/test/cases/signed_id_test.rb
@@ -3,20 +3,35 @@
 require "cases/helper"
 require "models/account"
 require "models/company"
+require "models/toy"
+require "models/matey"
 
 SIGNED_ID_VERIFIER_TEST_SECRET = "This is normally set by the railtie initializer when used with Rails!"
 
 ActiveRecord::Base.signed_id_verifier_secret = SIGNED_ID_VERIFIER_TEST_SECRET
 
 class SignedIdTest < ActiveRecord::TestCase
-  fixtures :accounts
+  fixtures :accounts, :toys
 
-  setup { @account = Account.first }
+  setup do
+    @account = Account.first
+    @toy = Toy.first
+  end
 
   test "find signed record" do
     assert_equal @account, Account.find_signed(@account.signed_id)
   end
 
+  test "find signed record with custom primary key" do
+    assert_equal @toy, Toy.find_signed(@toy.signed_id)
+  end
+
+  test "raise UnknownPrimaryKey when model have no primary key" do
+    assert_raises(ActiveRecord::UnknownPrimaryKey) do
+      Matey.find_signed("this will not be even verified")
+    end
+  end
+
   test "find signed record with a bang" do
     assert_equal @account, Account.find_signed!(@account.signed_id)
   end