diff --git a/lib/batch_loader.rb b/lib/batch_loader.rb
index bfc7819..99d3b5c 100644
--- a/lib/batch_loader.rb
+++ b/lib/batch_loader.rb
@@ -77,23 +77,29 @@ def __ensure_batched
     return if __executor_proxy.value_loaded?(item: @item)
 
     items = __executor_proxy.list_items
-    loader =  -> (item, value = (no_value = true; nil), &block) do
+    loader = __loader
+    @batch_block.call(items, loader)
+    items.each do |item|
+      next if __executor_proxy.value_loaded?(item: item)
+      loader.call(item, @default_value)
+    end
+    __executor_proxy.delete(items: items)
+  end
+
+  def __loader
+    mutex = Mutex.new
+    -> (item, value = (no_value = true; nil), &block) do
       if no_value && !block
         raise ArgumentError, "Please pass a value or a block"
       elsif block && !no_value
         raise ArgumentError, "Please pass a value or a block, not both"
       end
 
-      next_value = block ? block.call(__executor_proxy.loaded_value(item: item)) : value
-      __executor_proxy.load(item: item, value: next_value)
-    end
-
-    @batch_block.call(items, loader)
-    items.each do |item|
-      next if __executor_proxy.value_loaded?(item: item)
-      loader.call(item, @default_value)
+      mutex.synchronize do
+        next_value = block ? block.call(__executor_proxy.loaded_value(item: item)) : value
+        __executor_proxy.load(item: item, value: next_value)
+      end
     end
-    __executor_proxy.delete(items: items)
   end
 
   def __singleton_class
diff --git a/spec/batch_loader_spec.rb b/spec/batch_loader_spec.rb
index 9705a52..778ba57 100644
--- a/spec/batch_loader_spec.rb
+++ b/spec/batch_loader_spec.rb
@@ -110,7 +110,7 @@
       slow_executor_proxy = SlowExecutorProxy.new([], &batch_block)
       lazy = BatchLoader.new(item: 1, executor_proxy: slow_executor_proxy).batch(default_value: [], &batch_block)
 
-      expect(lazy).to eq([1, 2])
+      expect(lazy).to match_array([1, 2])
     end
 
     it 'supports alternative default values' do