diff --git a/mail-notify.gemspec b/mail-notify.gemspec
index 319c586..dc25bee 100644
--- a/mail-notify.gemspec
+++ b/mail-notify.gemspec
@@ -41,6 +41,6 @@ Gem::Specification.new do |spec|
   spec.add_dependency "activesupport", ">= 5.2.4.6"
   spec.add_dependency "actionpack", ">= 5.2.7.1"
   spec.add_dependency "actionview", ">= 5.2.7.1"
-  spec.add_dependency "notifications-ruby-client", "~> 5.1"
+  spec.add_dependency "notifications-ruby-client", ">= 5.1", "< 7.0"
   spec.add_dependency "rack", ">= 2.1.4.1"
 end