--- a/dom/media/ogg/OpusParser.cpp
+++ b/dom/media/ogg/OpusParser.cpp
@@ -81,27 +81,29 @@ bool OpusParser::DecodeHeader(unsigned c
         OPUS_LOG(LogLevel::Debug, ("Invalid Opus file: too many channels (%d) for"
                            " mapping family 0.", mChannels));
         return false;
       }
       mStreams = 1;
       mCoupledStreams = mChannels - 1;
       mMappingTable[0] = 0;
       mMappingTable[1] = 1;
-    } else if (mChannelMapping == 1) {
-      // Currently only up to 8 channels are defined for mapping family 1
+    } else if (mChannelMapping == 1 || mChannelMapping == 255) {
+      // Currently only up to 8 channels are defined for mapping family 1 and
+      // 255.
       if (mChannels>8) {
         OPUS_LOG(LogLevel::Debug, ("Invalid Opus file: too many channels (%d) for"
                            " mapping family 1.", mChannels));
         return false;
       }
       if (aLength>static_cast<unsigned>(20+mChannels)) {
         mStreams = aData[19];
         mCoupledStreams = aData[20];
         int i;
+        MOZ_ASSERT(mChannels < 255);
         for (i=0; i<mChannels; i++)
           mMappingTable[i] = aData[21+i];
       } else {
         OPUS_LOG(LogLevel::Debug, ("Invalid Opus file: channel mapping %d,"
                            " but no channel mapping table", mChannelMapping));
         return false;
       }
     } else {