--- a/servo/ports/geckolib/glue.rs
+++ b/servo/ports/geckolib/glue.rs
@@ -1414,17 +1414,17 @@ pub extern "C" fn Servo_AssertTreeIsClea
 pub extern "C" fn Servo_StyleSet_FillKeyframesForName(raw_data: RawServoStyleSetBorrowed,
                                                       name: *const nsACString,
                                                       timing_function: *const nsTimingFunction,
                                                       style: ServoComputedValuesBorrowed,
                                                       keyframes: RawGeckoKeyframeListBorrowedMut) -> bool {
     use style::gecko_bindings::structs::Keyframe;
     use style::properties::LonghandIdSet;
 
-    let data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
+    let data = PerDocumentStyleData::from_ffi(raw_data).borrow();
     let name = unsafe { Atom::from(name.as_ref().unwrap().as_str_unchecked()) };
     let style_timing_function = unsafe { timing_function.as_ref().unwrap() };
     let style = ComputedValues::as_arc(&style);
 
     if let Some(ref animation) = data.stylist.animations().get(&name) {
        for step in &animation.steps {
           // Override timing_function if the keyframe has animation-timing-function.
           let timing_function = if let Some(val) = step.get_animation_timing_function() {