--- a/dom/media/wave/WaveDemuxer.cpp
+++ b/dom/media/wave/WaveDemuxer.cpp
@@ -422,17 +422,18 @@ WAVTrackDemuxer::Duration() const
 }
 
 TimeUnit
 WAVTrackDemuxer::Duration(int64_t aNumDataChunks) const
 {
   if (!mSamplesPerSecond) {
     return TimeUnit();
   }
-  const double usPerDataChunk = USECS_PER_S * mSamplesPerChunk /
+  const double usPerDataChunk = USECS_PER_S *
+                                static_cast<double>(mSamplesPerChunk) /
                                 mSamplesPerSecond;
   return TimeUnit::FromMicroseconds(aNumDataChunks * usPerDataChunk);
 }
 
 TimeUnit
 WAVTrackDemuxer::DurationFromBytes(uint32_t aNumBytes) const
 {
   if (!mSamplesPerSecond) {