diff --git a/twirp.gemspec b/twirp.gemspec
index 965b58d..c39f529 100644
--- a/twirp.gemspec
+++ b/twirp.gemspec
@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.required_ruby_version = '>= 1.9'
-  spec.add_runtime_dependency 'google-protobuf', '~> 3.0', '>= 3.7.0'
+  spec.add_runtime_dependency 'google-protobuf', '>= 3.7.0', '< 5'
   spec.add_runtime_dependency 'faraday', '< 3' # for clients
   spec.add_dependency 'rack', '>= 2.2.3' # for service server handling