diff --git a/lib/dry/schema/messages/yaml.rb b/lib/dry/schema/messages/yaml.rb
index 2accc320..e733362c 100644
--- a/lib/dry/schema/messages/yaml.rb
+++ b/lib/dry/schema/messages/yaml.rb
@@ -70,7 +70,9 @@ def self.flat_hash(hash, path = EMPTY_ARRAY, keys = {})
 
         # @api private
         def self.cache
-          @cache ||= Concurrent::Map.new { |h, k| h[k] = Concurrent::Map.new }
+          @cache ||= Concurrent::Map.new do |h, k|
+            h.compute_if_absent(k) { Concurrent::Map.new }
+          end
         end
 
         # @api private