diff --git a/gemfiles/activemodel-6.0.gemfile b/gemfiles/activemodel-6.0.gemfile
index dfdf34a..2686691 100644
--- a/gemfiles/activemodel-6.0.gemfile
+++ b/gemfiles/activemodel-6.0.gemfile
@@ -1,5 +1,6 @@
 source "https://rubygems.org"
 
 gem "activemodel", "~> 6.0.0"
+gem "concurrent-ruby", "< 1.3.5"
 
 gemspec path: "../"
diff --git a/gemfiles/activemodel-6.1.gemfile b/gemfiles/activemodel-6.1.gemfile
index 6df0947..b594952 100644
--- a/gemfiles/activemodel-6.1.gemfile
+++ b/gemfiles/activemodel-6.1.gemfile
@@ -1,5 +1,6 @@
 source "https://rubygems.org"
 
 gem "activemodel", "~> 6.1.0"
+gem "concurrent-ruby", "< 1.3.5"
 
 gemspec path: "../"
diff --git a/gemfiles/activemodel-7.0.gemfile b/gemfiles/activemodel-7.0.gemfile
index 4691b92..059c2a7 100644
--- a/gemfiles/activemodel-7.0.gemfile
+++ b/gemfiles/activemodel-7.0.gemfile
@@ -1,5 +1,6 @@
 source "https://rubygems.org"
 
 gem "activemodel", "~> 7.0.0"
+gem "concurrent-ruby", "< 1.3.5"
 
 gemspec path: "../"