diff --git a/README.md b/README.md
index 81ce7de..05b9d08 100644
--- a/README.md
+++ b/README.md
@@ -81,7 +81,7 @@ Once you have an initial orbit defined, you can get updated *position* (`r`) and
 * Only point objects are considered in a two-body environment.
 * Perturbations due to atmospheric drag, solar radiation, etc are not considered.
 * Nodal precession is not considered.
-* Ruby 2.1 or greater is required because STL Vector#cross_product was not defined until Ruby 2.1
+* Ruby 2.1.3 or greater is required because of a bug in STL `Vector#cross_product` which was present before that release.
 
 ## Specs
 
diff --git a/kepler.gemspec b/kepler.gemspec
index 8ba5efb..0bd740a 100644
--- a/kepler.gemspec
+++ b/kepler.gemspec
@@ -17,6 +17,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ["lib"]
+  spec.required_ruby_version = ">= 2.1.3"
 
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "rspec", "~> 3.0"