diff --git a/Gemfile.lock b/Gemfile.lock
index 6444507..6b77f67 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -117,7 +117,7 @@ GEM
     rb-fsevent (0.11.0)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rdoc (5.1.0)
+    rdoc (6.3.3)
     rexml (3.2.5)
     rspec (3.8.0)
       rspec-core (~> 3.8.0)
@@ -177,7 +177,7 @@ DEPENDENCIES
   listen (~> 3.1, >= 3.5.0)!
   minitest (~> 5.14, >= 5.14.2)!
   rack (~> 2, >= 2.0.4)!
-  rdoc (~> 5.1, >= 5.1.0)!
+  rdoc (~> 6.3, >= 5.1.0)!
   rspec (~> 3.7, >= 3.7.0)!
   rubocop (~> 0.52.1, >= 0.52.1)!
   simplecov (~> 0.14, >= 0.14.1)!
diff --git a/aemo.gemspec b/aemo.gemspec
index efd3e9c..98d3049 100644
--- a/aemo.gemspec
+++ b/aemo.gemspec
@@ -39,7 +39,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'listen', '~> 3.1', '>= 3.5.0'
   s.add_development_dependency 'minitest', '~> 5.14', '>= 5.14.2'
   s.add_development_dependency 'rack', '~> 2', '>= 2.0.4'
-  s.add_development_dependency 'rdoc', '~> 5.1', '>= 5.1.0'
+  s.add_development_dependency 'rdoc', '>= 5.1.0', '~> 6.3'
   s.add_development_dependency 'rspec', '~> 3.7', '>= 3.7.0'
   s.add_development_dependency 'rubocop', '~> 0.52.1', '>= 0.52.1'
   s.add_development_dependency 'simplecov', '~> 0.14', '>= 0.14.1'