diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index 62d5740d31a..027151af9b4 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -646,10 +646,9 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             device_pixel_ratio: dppx,
             initial_viewport: initial_viewport,
         };
-        let top_level_browsing_context_id = match self.root_pipeline {
-            Some(ref pipeline) => pipeline.top_level_browsing_context_id,
-            None => return warn!("Window resize without root pipeline."),
-        };
+        let top_level_browsing_context_id = self.root_pipeline.as_ref().map(|pipeline| {
+            pipeline.top_level_browsing_context_id
+        });
         let msg = ConstellationMsg::WindowSize(top_level_browsing_context_id, data, size_type);
 
         if let Err(e) = self.constellation_chan.send(msg) {
diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index 3db4c5c39f2..f6a47ba14b1 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -2551,14 +2551,16 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
     /// Called when the window is resized.
     fn handle_window_size_msg(&mut self,
-                              top_level_browsing_context_id: TopLevelBrowsingContextId,
+                              top_level_browsing_context_id: Option<TopLevelBrowsingContextId>,
                               new_size: WindowSizeData,
                               size_type: WindowSizeType)
     {
         debug!("handle_window_size_msg: {:?}", new_size.initial_viewport.to_untyped());
 
-        let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
-        self.resize_browsing_context(new_size, size_type, browsing_context_id);
+        if let Some(top_level_browsing_context_id) = top_level_browsing_context_id {
+            let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
+            self.resize_browsing_context(new_size, size_type, browsing_context_id);
+        }
 
         if let Some(resize_channel) = self.webdriver.resize_channel.take() {
             let _ = resize_channel.send(new_size);
diff --git a/components/script_traits/lib.rs b/components/script_traits/lib.rs
index 14e0472da60..eec50faf09b 100644
--- a/components/script_traits/lib.rs
+++ b/components/script_traits/lib.rs
@@ -678,7 +678,7 @@ pub enum ConstellationMsg {
     /// Request to traverse the joint session history of the provided browsing context.
     TraverseHistory(TopLevelBrowsingContextId, TraversalDirection),
     /// Inform the constellation of a window being resized.
-    WindowSize(TopLevelBrowsingContextId, WindowSizeData, WindowSizeType),
+    WindowSize(Option<TopLevelBrowsingContextId>, WindowSizeData, WindowSizeType),
     /// Requests that the constellation instruct layout to begin a new tick of the animation.
     TickAnimation(PipelineId, AnimationTickType),
     /// Dispatch a webdriver command