diff --git a/radiation.gemspec b/radiation.gemspec
index ee5b6e7..f7d4b79 100644
--- a/radiation.gemspec
+++ b/radiation.gemspec
@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
 
   spec.add_dependency "plusminus"
   spec.add_dependency "open-uri-cached"
-  spec.add_dependency "combinatorics"
+  spec.add_dependency "combinatorics", "~> 0.4"
   spec.add_dependency "linefit"
   spec.add_dependency "xml-simple"
   spec.add_dependency "thor"