diff --git a/content/media/webrtc/MediaEngineWebRTCVideo.cpp b/content/media/webrtc/MediaEngineWebRTCVideo.cpp
index 353572301e23..b4941d941d99 100644
--- a/content/media/webrtc/MediaEngineWebRTCVideo.cpp
+++ b/content/media/webrtc/MediaEngineWebRTCVideo.cpp
@@ -79,8 +79,8 @@ MediaEngineWebRTCVideoSource::DeliverFrame(
     return 0;
   }
 
-  MOZ_ASSERT(mWidth*mHeight*3/2 == size);
-  if (mWidth*mHeight*3/2 != size) {
+  if (mWidth*mHeight + 2*(((mWidth+1)/2)*((mHeight+1)/2)) != size) {
+    MOZ_ASSERT(false, "Wrong size frame in DeliverFrame!");
     return 0;
   }
 
@@ -93,14 +93,15 @@ MediaEngineWebRTCVideoSource::DeliverFrame(
   const uint8_t lumaBpp = 8;
   const uint8_t chromaBpp = 4;
 
+  // Take lots of care to round up!
   layers::PlanarYCbCrData data;
   data.mYChannel = frame;
   data.mYSize = IntSize(mWidth, mHeight);
-  data.mYStride = mWidth * lumaBpp/ 8;
-  data.mCbCrStride = mWidth * chromaBpp / 8;
+  data.mYStride = (mWidth * lumaBpp + 7)/ 8;
+  data.mCbCrStride = (mWidth * chromaBpp + 7) / 8;
   data.mCbChannel = frame + mHeight * data.mYStride;
-  data.mCrChannel = data.mCbChannel + mHeight * data.mCbCrStride / 2;
-  data.mCbCrSize = IntSize(mWidth/ 2, mHeight/ 2);
+  data.mCrChannel = data.mCbChannel + ((mHeight+1)/2) * data.mCbCrStride;
+  data.mCbCrSize = IntSize((mWidth+1)/ 2, (mHeight+1)/ 2);
   data.mPicX = 0;
   data.mPicY = 0;
   data.mPicSize = IntSize(mWidth, mHeight);