diff --git a/lib/rom/factory/sequences.rb b/lib/rom/factory/sequences.rb
index 9a76ce2..ccaad32 100644
--- a/lib/rom/factory/sequences.rb
+++ b/lib/rom/factory/sequences.rb
@@ -19,12 +19,15 @@ def self.[](relation)
 
       # @api private
       def initialize
+        @mutex = Mutex.new
         reset
       end
 
       # @api private
       def next(key)
-        registry[key] += 1
+        @mutex.synchronize do
+          registry[key] += 1
+        end
       end
 
       # @api private