diff --git a/Gemfile.lock b/Gemfile.lock
index beecc96..bfb3565 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -18,7 +18,7 @@ GEM
     multi_xml (0.6.0)
     power_assert (0.2.6)
     rake (13.0.1)
-    rdoc (5.1.0)
+    rdoc (6.3.2)
     terminal-table (1.8.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     test-unit (3.2.5)
diff --git a/op5util.gemspec b/op5util.gemspec
index ce91935..d2d90a3 100644
--- a/op5util.gemspec
+++ b/op5util.gemspec
@@ -18,7 +18,7 @@ spec = Gem::Specification.new do |s|
   s.bindir = 'bin'
   s.executables << 'op5util'
   s.add_development_dependency 'rake', '~> 13'
-  s.add_development_dependency 'rdoc', '~> 0'
+  s.add_development_dependency 'rdoc', '~> 6'
   s.add_development_dependency 'test-unit', '~> 0'
   s.add_runtime_dependency('colorize', '0.8.1')
   s.add_runtime_dependency('gli', '2.16.1')