diff --git a/lib/src/ast/sass/expression/list.dart b/lib/src/ast/sass/expression/list.dart
index ee975ff2..3bf4544e 100644
--- a/lib/src/ast/sass/expression/list.dart
+++ b/lib/src/ast/sass/expression/list.dart
@@ -48,14 +48,14 @@ class ListExpression implements Expression {
       if (expression.isBracketed) return false;
       return separator == ListSeparator.comma
           ? separator == ListSeparator.comma
-          : separator != ListSeparator.unknown;
+          : separator != ListSeparator.undecided;
     }
 
     if (separator != ListSeparator.space) return false;
 
-    if (element is UnaryOperatorExpression) {
-      return element.operator == UnaryOperator.plus ||
-          element.operator == UnaryOperator.minus;
+    if (expression is UnaryOperatorExpression) {
+      return expression.operator == UnaryOperator.plus ||
+          expression.operator == UnaryOperator.minus;
     }
 
     // TODO: handle binary operations.
diff --git a/lib/src/value.dart b/lib/src/value.dart
index 2d9c4a32..05276ceb 100644
--- a/lib/src/value.dart
+++ b/lib/src/value.dart
@@ -4,6 +4,7 @@
 
 import 'value/boolean.dart';
 import 'value/identifier.dart';
+import 'visitor/css/serialize.dart';
 import 'visitor/value.dart';
 
 export 'value/boolean.dart';
diff --git a/lib/src/value/list.dart b/lib/src/value/list.dart
index 1f67aee3..4076aa45 100644
--- a/lib/src/value/list.dart
+++ b/lib/src/value/list.dart
@@ -45,11 +45,11 @@ class SassList extends Value {
 
   bool _elementNeedsParens(Value value) {
     if (value is SassList) {
-      if (expression.contents.length < 2) return false;
-      if (expression.isBracketed) return false;
+      if (value.contents.length < 2) return false;
+      if (value.isBracketed) return false;
       return separator == ListSeparator.comma
           ? separator == ListSeparator.comma
-          : separator != ListSeparator.unknown;
+          : separator != ListSeparator.undecided;
     }
     return false;
   }