diff --git a/lib/prometheus/client/metric.rb b/lib/prometheus/client/metric.rb
index e2dd91e6..5c8b5777 100644
--- a/lib/prometheus/client/metric.rb
+++ b/lib/prometheus/client/metric.rb
@@ -61,8 +61,8 @@ def label_set_for(labels)
         @validator.validate(labels)
       end
 
-      def synchronize(&block)
-        @mutex.synchronize(&block)
+      def synchronize
+        @mutex.synchronize { yield }
       end
     end
   end