--- a/dom/media/MediaDecoder.cpp
+++ b/dom/media/MediaDecoder.cpp
@@ -1031,17 +1031,18 @@ MediaDecoder::GetStatistics()
 
 void
 MediaDecoder::ComputePlaybackRate()
 {
   MOZ_ASSERT(NS_IsMainThread());
   MOZ_ASSERT(mResource);
 
   int64_t length = mResource->GetLength();
-  if (!IsNaN(mDuration) && !mozilla::IsInfinite<double>(mDuration)
+  if (mozilla::IsFinite<double>(mDuration)
+      && mDuration > 0
       && length >= 0) {
     mPlaybackRateReliable = true;
     mPlaybackBytesPerSecond = length / mDuration;
     return;
   }
 
   bool reliable = false;
   mPlaybackBytesPerSecond = mPlaybackStatistics->GetRateAtLastStop(&reliable);