--- a/dom/animation/AnimationEffectReadOnly.cpp
+++ b/dom/animation/AnimationEffectReadOnly.cpp
@@ -229,17 +229,17 @@ AnimationEffectReadOnly::GetComputedTimi
       break;
     case PlaybackDirection::Alternate:
       thisIterationReverse = (result.mCurrentIteration & 1) == 1;
       break;
     case PlaybackDirection::Alternate_reverse:
       thisIterationReverse = (result.mCurrentIteration & 1) == 0;
       break;
     default:
-      MOZ_ASSERT(true, "Unknown PlaybackDirection type");
+      MOZ_ASSERT_UNREACHABLE("Unknown PlaybackDirection type");
   }
   if (thisIterationReverse) {
     progress = 1.0 - progress;
   }
 
   // Calculate the 'before flag' which we use when applying step timing
   // functions.
   if ((result.mPhase == ComputedTiming::AnimationPhase::After &&