diff --git a/Gemfile.lock b/Gemfile.lock
index 0e17538..0a3fc55 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -2,7 +2,7 @@ PATH
   remote: .
   specs:
     mrml-rails (0.1.0)
-      actionmailer
+      actionmailer (>= 3.1)
       mrml (~> 0.2.0)
       railties (>= 3.1)
 
diff --git a/mrml-rails.gemspec b/mrml-rails.gemspec
index 1e5585d..e02d2f5 100644
--- a/mrml-rails.gemspec
+++ b/mrml-rails.gemspec
@@ -20,9 +20,9 @@ Gem::Specification.new do |spec|
   spec.metadata['source_code_uri'] = 'https://github.com/oddcamp/mrml-rails'
   spec.metadata['changelog_uri'] = 'https://github.com/oddcamp/mrml-rails/releases'
 
-  spec.add_runtime_dependency 'actionmailer'
+  spec.add_runtime_dependency 'actionmailer', '>= 3.1'
   spec.add_runtime_dependency 'mrml', '~> 0.2.0'
-  spec.add_runtime_dependency 'railties', ['>= 3.1']
+  spec.add_runtime_dependency 'railties', '>= 3.1'
   spec.add_development_dependency 'rspec', '~> 3.10'
   spec.add_development_dependency 'rspec-rails', '~> 4.0', '>= 4.0.2'
 end