--- a/dom/animation/KeyframeEffect.cpp
+++ b/dom/animation/KeyframeEffect.cpp
@@ -237,19 +237,20 @@ KeyframeEffectReadOnly::GetComputedTimin
   ComputedTiming result;
 
   if (aTiming.mDuration) {
     MOZ_ASSERT(aTiming.mDuration.ref() >= zeroDuration,
                "Iteration duration should be positive");
     result.mDuration = aTiming.mDuration.ref();
   }
 
-  result.mIterations = IsNaN(aTiming.mIterations) || aTiming.mIterations < 0.0f ?
-                       1.0f :
-                       aTiming.mIterations;
+  MOZ_ASSERT(aTiming.mIterations >= 0.0 && !IsNaN(aTiming.mIterations),
+             "mIterations should be nonnegative & finite, as ensured by "
+             "ValidateIterations or CSSParser");
+  result.mIterations = aTiming.mIterations;
   MOZ_ASSERT(aTiming.mIterationStart >= 0.0,
              "mIterationStart should be nonnegative, as ensured by "
              "ValidateIterationStart");
   result.mIterationStart = aTiming.mIterationStart;
 
   result.mActiveDuration = ActiveDuration(result.mDuration, result.mIterations);
   result.mEndTime = aTiming.mDelay + result.mActiveDuration +
                     aTiming.mEndDelay;