diff --git a/lib/rspec-puppet-facts.rb b/lib/rspec-puppet-facts.rb
index 0effa15..2d9e1e4 100644
--- a/lib/rspec-puppet-facts.rb
+++ b/lib/rspec-puppet-facts.rb
@@ -2,6 +2,7 @@
 require 'facter'
 require 'facterdb'
 require 'json'
+require 'deep_merge'
 
 # The purpose of this module is to simplify the Puppet
 # module's RSpec tests by looping through all supported
@@ -229,9 +230,9 @@ def self.with_custom_facts(os, facts)
       next if fact[:options][:exclude] && fact[:options][:exclude].include?(os)
 
       if fact[:value].respond_to?(:call)
-        facts = facts.deep_merge({name.to_sym => fact[:value].call(os, facts)})
+        facts = facts.deep_merge!({name.to_sym => fact[:value].call(os, facts)})
       else
-        facts = facts.deep_merge({name.to_sym => fact[:value]})
+        facts = facts.deep_merge!({name.to_sym => fact[:value]})
       end
     end
 
@@ -408,11 +409,3 @@ def self.facter_version_for_puppet_version(puppet_version)
     :default => RspecPuppetFacts.facter_version_for_puppet_version(Puppet.version)
   c.add_setting :facterdb_string_keys, :default => false
 end
-
-# monkey patch the Hash Class to add deep_merge
-class ::Hash
-  def deep_merge(second)
-    merger = proc { |_, v1, v2| Hash === v1 && Hash === v2 ? v1.merge(v2, &merger) : Array === v1 && Array === v2 ? v1 | v2 : [:undefined, nil, :nil].include?(v2) ? v1 : v2 }
-    merge(second.to_h, &merger)
-  end
-end
diff --git a/rspec-puppet-facts.gemspec b/rspec-puppet-facts.gemspec
index 6698f7a..97bddcb 100644
--- a/rspec-puppet-facts.gemspec
+++ b/rspec-puppet-facts.gemspec
@@ -26,6 +26,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'voxpupuli-rubocop', '~> 2.0'
 
   s.add_runtime_dependency 'facter'
+  s.add_runtime_dependency 'deep_merge', '~> 1.2'
   s.add_runtime_dependency 'facterdb', '>= 0.5.0'
   s.add_runtime_dependency 'puppet'
 end