diff --git a/middleman-autoprefixer.gemspec b/middleman-autoprefixer.gemspec
index 2c4d51a..c96c5d4 100644
--- a/middleman-autoprefixer.gemspec
+++ b/middleman-autoprefixer.gemspec
@@ -17,7 +17,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ['lib']
 
   spec.add_runtime_dependency 'middleman-core',     '>= 3.3.3'
-  spec.add_runtime_dependency 'autoprefixer-rails', '>= 9.1.4'
+  spec.add_runtime_dependency 'autoprefixer-rails', '~> 9.1'
 
   spec.add_development_dependency 'bundler',        '>= 1.14'
   spec.add_development_dependency 'rake',           '>= 10.3'