diff --git a/README.md b/README.md
index 9fc4fac..69767b2 100644
--- a/README.md
+++ b/README.md
@@ -290,7 +290,11 @@ These are not required automatically. You must require `loofah/helpers` to use t
 
 Unsurprisingly:
 
-* gem install loofah
+> gem install loofah
+
+Requirements:
+
+* Ruby >= 2.5
 
 
 ## Support
diff --git a/loofah.gemspec b/loofah.gemspec
index ca822fb..d84ec42 100644
--- a/loofah.gemspec
+++ b/loofah.gemspec
@@ -42,6 +42,8 @@ Gem::Specification.new do |spec|
     ] + Dir.glob("lib/**/*.*")
   end
 
+  spec.required_ruby_version = ">= 2.5.0"
+
   spec.add_runtime_dependency("crass", ["~> 1.0.2"])
-  spec.add_runtime_dependency("nokogiri", [">= 1.5.9"])
+  spec.add_runtime_dependency("nokogiri", [">= 1.12.0"])
 end