--- a/dom/media/mediasource/ContainerParser.cpp
+++ b/dom/media/mediasource/ContainerParser.cpp
@@ -398,20 +398,20 @@ private:
         for (const auto& boxType : validBoxes) {
           if (type == boxType) {
             mValid = true;
             break;
           }
         }
         if (!mValid) {
           // No point continuing.
-          mLastInvalidBox[0] = typec[3];
-          mLastInvalidBox[1] = typec[2];
-          mLastInvalidBox[2] = typec[1];
-          mLastInvalidBox[3] = typec[0];
+          mLastInvalidBox[0] = typec[0];
+          mLastInvalidBox[1] = typec[1];
+          mLastInvalidBox[2] = typec[2];
+          mLastInvalidBox[3] = typec[3];
           mLastInvalidBox[4] = '\0';
           break;
         }
         if (mInitOffset.isNothing() &&
             mp4_demuxer::AtomType(type) == initAtom) {
           mInitOffset = Some(reader.Offset());
         }
         if (mMediaOffset.isNothing() &&