diff --git a/app/models/principal.rb b/app/models/principal.rb
index 1ef191796..fe5ef35d2 100644
--- a/app/models/principal.rb
+++ b/app/models/principal.rb
@@ -103,8 +103,8 @@ class Principal < ApplicationRecord
   end)
   # Principals that are not members of projects
   scope :not_member_of, (lambda do |projects|
-    projects = [projects] unless projects.is_a?(Array)
-    if projects.empty?
+    projects = [projects] if projects.is_a?(Project)
+    if projects.blank?
       where("1=0")
     else
       ids = projects.map(&:id)
diff --git a/test/unit/principal_test.rb b/test/unit/principal_test.rb
index f36ace7be..8e8a4bdc1 100644
--- a/test/unit/principal_test.rb
+++ b/test/unit/principal_test.rb
@@ -76,6 +76,12 @@ class PrincipalTest < ActiveSupport::TestCase
     end
   end
 
+  def test_not_member_of_scope_should_accept_active_record_relation
+    projects = Project.where(id: [1, 2])
+    expected = (Principal.all - projects.map(&:memberships).flatten.map(&:principal)).sort
+    assert_equal expected, Principal.not_member_of(projects).sort
+  end
+
   def test_not_member_of_scope_should_be_empty_for_no_projects
     assert_equal [], Principal.not_member_of([]).sort
   end