diff --git a/activesupport/lib/active_support/testing/time_helpers.rb b/activesupport/lib/active_support/testing/time_helpers.rb
index fbab62b7b6f4b..8254111fe747d 100644
--- a/activesupport/lib/active_support/testing/time_helpers.rb
+++ b/activesupport/lib/active_support/testing/time_helpers.rb
@@ -11,7 +11,9 @@ class SimpleStubs # :nodoc:
       Stub = Struct.new(:object, :method_name, :original_method)
 
       def initialize
-        @stubs = Concurrent::Map.new { |h, k| h[k] = {} }
+        @stubs = Concurrent::Map.new do |h, k|
+          h.compute_if_absent(k) { {} }
+        end
       end
 
       # Stubs object.method_name with the given block