diff --git a/lib/haml/attribute_builder.rb b/lib/haml/attribute_builder.rb
index d50e8467f..fb3271e09 100644
--- a/lib/haml/attribute_builder.rb
+++ b/lib/haml/attribute_builder.rb
@@ -105,7 +105,7 @@ def flatten_attributes(attributes)
             if k.nil?
               flattened[key] = v
             else
-              flattened["#{key}-#{k.to_s.gsub(/_/, '-')}"] = v
+              flattened["#{key}-#{k.to_s.tr('_', '-')}"] = v
             end
           end
         else