diff --git a/components/style/gecko_bindings/sugar/ns_style_auto_array.rs b/components/style/gecko_bindings/sugar/ns_style_auto_array.rs
index 9c41a0b6ec4..b20ee735631 100644
--- a/components/style/gecko_bindings/sugar/ns_style_auto_array.rs
+++ b/components/style/gecko_bindings/sugar/ns_style_auto_array.rs
@@ -7,7 +7,7 @@
 use gecko_bindings::bindings::Gecko_EnsureStyleAnimationArrayLength;
 use gecko_bindings::structs::nsStyleAutoArray;
 use std::iter::{once, Chain, Once, IntoIterator};
-use std::ops::Index;
+use std::ops::{Index, IndexMut};
 use std::slice::{Iter, IterMut};
 
 impl<T> Index<usize> for nsStyleAutoArray<T> {
@@ -23,6 +23,18 @@ impl<T> Index<usize> for nsStyleAutoArray<T> {
     }
 }
 
+impl<T> IndexMut<usize> for nsStyleAutoArray<T> {
+    fn index_mut(&mut self, index: usize) -> &mut T {
+        if index > self.len() {
+            panic!("out of range")
+        }
+        match index {
+            0 => &mut self.mFirstElement,
+            _ => &mut self.mOtherElements[index - 1],
+        }
+    }
+}
+
 impl<T> nsStyleAutoArray<T> {
     /// Mutably iterate over the array elements.
     pub fn iter_mut(&mut self) -> Chain<Once<&mut T>, IterMut<T>> {