Index: json.py
===================================================================
--- json.py	(revision 2097)
+++ json.py	(working copy)
@@ -457,14 +457,17 @@
     """
     Returns the review request with the specified ID.
     """
-    review_request = get_object_or_404(ReviewRequest, pk=review_request_id)
+    try:
+        review_request = ReviewRequest.objects.get(pk=review_request_id)
+        if not review_request.is_accessible_by(request.user):
+            return WebAPIResponseError(request, PERMISSION_DENIED)
+        else:
+            return WebAPIResponse(request, {'review_request': review_request})
+    
+    except ReviewRequest.DoesNotExist:
+        return WebAPIResponseError(request, INVALID_CHANGE_NUMBER)
 
-    if not review_request.is_accessible_by(request.user):
-        return WebAPIResponseError(request, PERMISSION_DENIED)
 
-    return WebAPIResponse(request, {'review_request': review_request})
-
-
 @webapi_check_login_required
 def review_request_last_update(request, review_request_id):
     """