diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index 851140e3929..af957f2beca 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -53,6 +53,7 @@ use script_traits::UntrustedNodeAddress;
 use util::geometry::Au;
 use util::namespace;
 use util::str::DOMString;
+use util::task_state;
 use selectors::parser::{Selector, AttrSelector, NamespaceConstraint};
 use selectors::parser::parse_author_origin_selector_list_from_str;
 use selectors::matching::matches;
@@ -222,7 +223,7 @@ pub struct LayoutData {
 unsafe impl Send for LayoutData {}
 
 pub struct LayoutDataRef {
-    pub data_cell: RefCell<Option<LayoutData>>,
+    data_cell: RefCell<Option<LayoutData>>,
 }
 
 no_jsmanaged_fields!(LayoutDataRef);
@@ -236,7 +237,8 @@ impl LayoutDataRef {
 
     /// Sends layout data, if any, back to the layout task to be destroyed.
     pub fn dispose(&self) {
-        if let Some(mut layout_data) = mem::replace(&mut *self.borrow_mut(), None) {
+        debug_assert!(task_state::get().is_script());
+        if let Some(mut layout_data) = mem::replace(&mut *self.data_cell.borrow_mut(), None) {
             let layout_chan = layout_data.chan.take();
             match layout_chan {
                 None => {}
@@ -248,18 +250,20 @@ impl LayoutDataRef {
         }
     }
 
-    /// Borrows the layout data immutably, *asserting that there are no mutators*. Bad things will
+    /// Borrows the layout data immutably, *assuming that there are no mutators*. Bad things will
     /// happen if you try to mutate the layout data while this is held. This is the only thread-
     /// safe layout data accessor.
     #[inline]
     #[allow(unsafe_code)]
     pub unsafe fn borrow_unchecked(&self) -> *const Option<LayoutData> {
+        debug_assert!(task_state::get().is_layout());
         self.data_cell.as_unsafe_cell().get() as *const _
     }
 
     /// Borrows the layout data immutably. This function is *not* thread-safe.
     #[inline]
     pub fn borrow<'a>(&'a self) -> Ref<'a,Option<LayoutData>> {
+        debug_assert!(task_state::get().is_layout());
         self.data_cell.borrow()
     }
 
@@ -270,6 +274,7 @@ impl LayoutDataRef {
     /// on it. This has already resulted in one bug!
     #[inline]
     pub fn borrow_mut<'a>(&'a self) -> RefMut<'a,Option<LayoutData>> {
+        debug_assert!(task_state::get().is_layout());
         self.data_cell.borrow_mut()
     }
 }