diff --git a/src/parser.cc b/src/parser.cc
index 74f69970e3..cff56a3951 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -1585,13 +1585,15 @@ class ThisNamedPropertyAssigmentFinder : public ParserFinder {
   }
 
   void HandleThisPropertyAssignment(Scope* scope, Assignment* assignment) {
-    // Check that the property assigned to is a named property.
+    // Check that the property assigned to is a named property, which is not
+    // __proto__.
     Property* property = assignment->target()->AsProperty();
     ASSERT(property != NULL);
     Literal* literal = property->key()->AsLiteral();
     uint32_t dummy;
     if (literal != NULL &&
         literal->handle()->IsString() &&
+        !String::cast(*(literal->handle()))->Equals(Heap::Proto_symbol()) &&
         !String::cast(*(literal->handle()))->AsArrayIndex(&dummy)) {
       Handle<String> key = Handle<String>::cast(literal->handle());
 
diff --git a/test/mjsunit/regress/regress-646.js b/test/mjsunit/regress/regress-646.js
new file mode 100644
index 0000000000..b862350f99
--- /dev/null
+++ b/test/mjsunit/regress/regress-646.js
@@ -0,0 +1,33 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Regression test for http://code.google.com/p/v8/issues/detail?id=646.
+
+function f() { this.__proto__ = 42 }
+var count = 0;
+for (var x in new f()) count++;
+assertEquals(0, count);