diff --git a/irb.gemspec b/irb.gemspec
index af14713f6..3a6d80094 100644
--- a/irb.gemspec
+++ b/irb.gemspec
@@ -45,4 +45,5 @@ Gem::Specification.new do |spec|
   spec.add_dependency "reline", ">= 0.4.2"
   spec.add_dependency "rdoc", ">= 4.0.0"
   spec.add_dependency "pp", ">= 0.6.0"
+  spec.add_dependency "tsort"
 end