diff --git a/mailcatcher.gemspec b/mailcatcher.gemspec
index ceadb0f..696231b 100644
--- a/mailcatcher.gemspec
+++ b/mailcatcher.gemspec
@@ -30,7 +30,7 @@ Gem::Specification.new do |s|
 
   s.required_ruby_version = ">= 1.9.3"
 
-  s.add_dependency "activesupport", ">= 4.0.0", "< 5"
+  s.add_dependency "activesupport", "~> 4.0"
   s.add_dependency "eventmachine", "1.0.9.1"
   s.add_dependency "mail", "~> 2.3"
   s.add_dependency "sinatra", "~> 1.2"