--- a/layout/style/nsTransitionManager.cpp
+++ b/layout/style/nsTransitionManager.cpp
@@ -652,19 +652,18 @@ nsTransitionManager::ConsiderStartingTra
   timing.mDuration.SetAsUnrestrictedDouble() = duration;
   timing.mDelay = TimeDuration::FromMilliseconds(delay);
   timing.mIterations = 1.0;
   timing.mDirection = dom::PlaybackDirection::Normal;
   timing.mFill = dom::FillMode::Backwards;
 
   RefPtr<ElementPropertyTransition> pt =
     new ElementPropertyTransition(aElement->OwnerDoc(), aElement,
-                                  aNewStyleContext->GetPseudoType(), timing);
-  pt->mStartForReversingTest = startForReversingTest;
-  pt->mReversePortion = reversePortion;
+                                  aNewStyleContext->GetPseudoType(), timing,
+                                  startForReversingTest, reversePortion);
 
   AnimationProperty& prop = *pt->Properties().AppendElement();
   prop.mProperty = aProperty;
 
   AnimationPropertySegment& segment = *prop.mSegments.AppendElement();
   segment.mFromValue = startValue;
   segment.mToValue = endValue;
   segment.mFromKey = 0;

--- a/layout/style/nsTransitionManager.h
+++ b/layout/style/nsTransitionManager.h
@@ -33,18 +33,22 @@ struct StyleTransition;
 
 namespace mozilla {
 
 struct ElementPropertyTransition : public dom::KeyframeEffectReadOnly
 {
   ElementPropertyTransition(nsIDocument* aDocument,
                             dom::Element* aTarget,
                             nsCSSPseudoElements::Type aPseudoType,
-                            const TimingParams &aTiming)
+                            const TimingParams &aTiming,
+                            StyleAnimationValue aStartForReversingTest,
+                            double aReversePortion)
     : dom::KeyframeEffectReadOnly(aDocument, aTarget, aPseudoType, aTiming)
+    , mStartForReversingTest(aStartForReversingTest)
+    , mReversePortion(aReversePortion)
   { }
 
   ElementPropertyTransition* AsTransition() override { return this; }
   const ElementPropertyTransition* AsTransition() const override
   {
     return this;
   }