diff --git a/mosql.gemspec b/mosql.gemspec
index 8089b30..6e51fe0 100644
--- a/mosql.gemspec
+++ b/mosql.gemspec
@@ -17,7 +17,7 @@ Gem::Specification.new do |gem|
   gem.version       = MoSQL::VERSION
 
   gem.add_runtime_dependency "sequel"
-  gem.add_runtime_dependency "pg"
+  gem.add_runtime_dependency "pg", "~> 0.21"
   gem.add_runtime_dependency "rake"
   gem.add_runtime_dependency "log4r"
   gem.add_runtime_dependency "json"