--- a/testing/geckodriver/src/marionette.rs
+++ b/testing/geckodriver/src/marionette.rs
@@ -735,40 +735,40 @@ impl MarionetteSession {
 
                 WebDriverResponse::ElementRect(RectResponse::new(x, y, width, height))
             },
             FullscreenWindow | MaximizeWindow | GetWindowRect |
             MaximizeWindow | SetWindowRect(_) => {
                 let width = try_opt!(
                     try_opt!(resp.result.find("width"),
                              ErrorStatus::UnknownError,
-                             "Failed to find width field").as_u64(),
+                             "Failed to find width field").as_f64(),
                     ErrorStatus::UnknownError,
-                    "Failed to interpret width as integer");
+                    "Failed to interpret width as float");
 
                 let height = try_opt!(
                     try_opt!(resp.result.find("height"),
                              ErrorStatus::UnknownError,
-                             "Failed to find height field").as_u64(),
+                             "Failed to find height field").as_f64(),
                     ErrorStatus::UnknownError,
-                    "Failed to interpret height as integer");
+                    "Failed to interpret height as float");
 
                 let x = try_opt!(
                     try_opt!(resp.result.find("x"),
                              ErrorStatus::UnknownError,
-                             "Failed to find x field").as_i64(),
+                             "Failed to find x field").as_f64(),
                     ErrorStatus::UnknownError,
-                    "Failed to interpret x as integer");
+                    "Failed to interpret x as float");
 
                 let y = try_opt!(
                     try_opt!(resp.result.find("y"),
                              ErrorStatus::UnknownError,
-                             "Failed to find y field").as_i64(),
+                             "Failed to find y field").as_f64(),
                     ErrorStatus::UnknownError,
-                    "Failed to interpret y as integer");
+                    "Failed to interpret y as float");
 
                 WebDriverResponse::WindowRect(RectResponse::new(x, y, width, height))
             },
             GetCookies => {
                 let cookies = try!(self.process_cookies(&resp.result));
                 WebDriverResponse::Cookie(CookieResponse::new(cookies))
             },
             GetNamedCookie(ref name) => {