diff --git a/README.md b/README.md
index 439faf4..6b70759 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@ A tree structure for Mongoid documents using the materialized path pattern
 
 ## Requirements
 
-* mongoid (>= 4.0, < 6.0)
+* mongoid (>= 4.0, < 7.0)
 
 For a mongoid 3.x compatible version, please use mongoid-tree 1.0.x,
 for a mongoid 2.x compatible version, please use mongoid-tree 0.7.x.
diff --git a/lib/mongoid/tree.rb b/lib/mongoid/tree.rb
index 3d8fb49..b559d3c 100644
--- a/lib/mongoid/tree.rb
+++ b/lib/mongoid/tree.rb
@@ -87,7 +87,17 @@ module Tree
     included do
       has_many :children, :class_name => self.name, :foreign_key => :parent_id, :inverse_of => :parent, :validate => false
 
-      belongs_to :parent, :class_name => self.name, :inverse_of => :children, :index => true, :validate => false
+      options = {
+        :class_name => self.name,
+        :inverse_of => :children,
+        :index => true,
+        :validate => false,
+        :optional => true
+      }
+
+      options.delete(:optional) if Gem::Version.new(Mongoid::VERSION) < Gem::Version.new('6.0.0.beta')
+
+      belongs_to :parent, options
 
       field :parent_ids, :type => Array, :default => []
       index :parent_ids => 1
diff --git a/mongoid-tree.gemspec b/mongoid-tree.gemspec
index 6e46b3f..feb1af8 100644
--- a/mongoid-tree.gemspec
+++ b/mongoid-tree.gemspec
@@ -12,7 +12,7 @@ Gem::Specification.new do |s|
 
   s.files         = Dir.glob('{lib,spec}/**/*') + %w(LICENSE README.md Rakefile Gemfile .rspec)
 
-  s.add_runtime_dependency('mongoid', ['< 6.0', '>= 4.0'])
+  s.add_runtime_dependency('mongoid', ['< 7.0', '>= 4.0'])
   s.add_development_dependency('rake', ['>= 0.9.2'])
   s.add_development_dependency('rspec', ['~> 3.0'])
   s.add_development_dependency('yard', ['~> 0.8'])