diff --git a/lib/src/node/value/boolean.dart b/lib/src/node/value/boolean.dart
index b069913e..d4b800fd 100644
--- a/lib/src/node/value/boolean.dart
+++ b/lib/src/node/value/boolean.dart
@@ -16,7 +16,7 @@ import '../utils.dart';
 /// to have a constructor injected into their inheritance chain so that
 /// `instanceof` works properly.
 final Function booleanConstructor = () {
-  var constructor = allowInterop(([_]) {
+  var constructor = allowInterop(([dynamic _]) {
     throw "new sass.types.Boolean() isn't allowed.\n"
         "Use sass.types.Boolean.TRUE or sass.types.Boolean.FALSE instead.";
   });
diff --git a/lib/src/node/value/null.dart b/lib/src/node/value/null.dart
index 97d2fc99..10c28d0b 100644
--- a/lib/src/node/value/null.dart
+++ b/lib/src/node/value/null.dart
@@ -16,7 +16,7 @@ import '../utils.dart';
 /// to have a constructor injected into their inheritance chain so that
 /// `instanceof` works properly.
 final Function nullConstructor = () {
-  var constructor = allowInterop(([_]) {
+  var constructor = allowInterop(([dynamic _]) {
     throw "new sass.types.Null() isn't allowed. Use sass.types.Null.NULL "
         "instead.";
   });