--- a/testing/webdriver/src/response.rs
+++ b/testing/webdriver/src/response.rs
@@ -21,22 +21,22 @@ pub enum WebDriverResponse {
 impl WebDriverResponse {
     pub fn to_json_string(self) -> String {
         use response::WebDriverResponse::*;
 
         let obj = match self {
             CloseWindow(ref x) => json::encode(&x.to_json()),
             Cookie(ref x) => json::encode(x),
             Cookies(ref x) => json::encode(x),
-            DeleteSession => Ok("{}".to_string()),
+            DeleteSession => Ok("null".to_string()),
             ElementRect(ref x) => json::encode(x),
             Generic(ref x) => json::encode(x),
             NewSession(ref x) => json::encode(x),
             Timeouts(ref x) => json::encode(x),
-            Void => Ok("{}".to_string()),
+            Void => Ok("null".to_string()),
             WindowRect(ref x) => json::encode(&x.to_json()),
         }.unwrap();
 
         match self {
             Generic(_) | Cookie(_) | Cookies(_) => obj,
             _ => {
                 let mut data = String::with_capacity(11 + obj.len());
                 data.push_str("{\"value\": ");