diff --git a/lib/sass/less.rb b/lib/sass/less.rb
index a7dcb898..6b43454b 100755
--- a/lib/sass/less.rb
+++ b/lib/sass/less.rb
@@ -140,6 +140,7 @@ WARNING
         def to_sass_tree
           return if hide_in_sass
           mixin = Sass::Tree::MixinDefNode.new(name, @params.map do |v|
+              v.value.flatten!
               [Sass::Script::Variable.new(v), v.value.to_sass_tree]
             end)
           rules.each {|r| mixin << r.to_sass_tree}
diff --git a/test/sass/less_conversion_test.rb b/test/sass/less_conversion_test.rb
index 2f170a2b..52bdf2f6 100755
--- a/test/sass/less_conversion_test.rb
+++ b/test/sass/less_conversion_test.rb
@@ -480,13 +480,13 @@ SCSS
 LESS
 
     assert_renders <<SCSS, <<LESS
-@mixin foo($a: 2px, $b: red) {
+@mixin foo($a: 2px + 3px, $b: red) {
   a: $a; }
 
 .bar {
   @include foo(5px); }
 SCSS
-.foo(@a: 2px, @b: #f00) {a: @a}
+.foo(@a: 2px + 3px, @b: #f00) {a: @a}
 .bar {.foo(5px);}
 LESS
   end