diff --git a/src/pyson.js b/src/pyson.js
index f775c98dbf..43aad4f2bd 100644
--- a/src/pyson.js
+++ b/src/pyson.js
@@ -179,7 +179,7 @@
                 'd': value.d,
             }, context[value.v.substring(0, idx)] || {});
         }
-        if (value.v in context) {
+        if ((value.v in context) && (context[value.v] !== undefined)) {
             return context[value.v];
         } else {
             return value.d;
diff --git a/tests/sao.js b/tests/sao.js
index 5f587b7718..98a358ab96 100644
--- a/tests/sao.js
+++ b/tests/sao.js
@@ -102,6 +102,9 @@
             "decode(Eval('test', 0))");
         QUnit.strictEqual(new Sao.PYSON.Decoder().decode(eval_), 0,
             "decode(Eval('test', 0))");
+        QUnit.strictEqual(
+            new Sao.PYSON.Decoder({test: undefined}).decode(eval_), 0,
+            "decode(Eval('test', 0))");
     });
 
     QUnit.test('PYSON Not', function() {