--- a/gfx/layers/composite/AsyncCompositionManager.cpp
+++ b/gfx/layers/composite/AsyncCompositionManager.cpp
@@ -584,19 +584,18 @@ struct StyleAnimationValueCompositePair 
 static StyleAnimationValue
 SampleValue(float aPortion, const Animation& aAnimation,
             const StyleAnimationValueCompositePair& aStart,
             const StyleAnimationValueCompositePair& aEnd,
             const StyleAnimationValue& aLastValue,
             uint64_t aCurrentIteration,
             const StyleAnimationValue& aUnderlyingValue)
 {
-  NS_ASSERTION(aStart.mValue.GetUnit() == aEnd.mValue.GetUnit() ||
-               aStart.mValue.GetUnit() == StyleAnimationValue::eUnit_None ||
-               aEnd.mValue.GetUnit() == StyleAnimationValue::eUnit_None,
+  NS_ASSERTION(aStart.mValue.IsNull() || aEnd.mValue.IsNull() ||
+               aStart.mValue.GetUnit() == aEnd.mValue.GetUnit(),
                "Must have same unit");
 
   StyleAnimationValue startValue =
     dom::KeyframeEffectReadOnly::CompositeValue(aAnimation.property(),
                                                 aStart.mValue,
                                                 aUnderlyingValue,
                                                 aStart.mComposite);
   StyleAnimationValue endValue =