--- a/dom/media/mediasource/ContainerParser.cpp
+++ b/dom/media/mediasource/ContainerParser.cpp
@@ -135,22 +135,24 @@ public:
     // ...
     // 0x18538067 // Segment (must be "unknown" size or contain a value large
                   // enough to include the Segment Information and Tracks
                   // elements that follow)
     // 0x1549a966 // -> Segment Info
     // 0x1654ae6b // -> One or more Tracks
 
     // 0x1a45dfa3 // EBML
-    if (aData->Length() >= 4 &&
-        (*aData)[0] == 0x1a && (*aData)[1] == 0x45 && (*aData)[2] == 0xdf &&
+    if (aData->Length() < 4) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+    if ((*aData)[0] == 0x1a && (*aData)[1] == 0x45 && (*aData)[2] == 0xdf &&
         (*aData)[3] == 0xa3) {
       return NS_OK;
     }
-    return NS_ERROR_NOT_AVAILABLE;
+    return MediaResult(NS_ERROR_FAILURE, RESULT_DETAIL("Invalid webm content"));
   }
 
   MediaResult IsMediaSegmentPresent(MediaByteBuffer* aData) override
   {
     ContainerParser::IsMediaSegmentPresent(aData);
     // XXX: This is overly primitive, needs to collect data as it's appended
     // to the SB and handle, rather than assuming everything is present in a
     // single aData segment.
@@ -158,28 +160,29 @@ public:
     // ...
     // DocType == "webm"
     // ...
     // 0x18538067 // Segment (must be "unknown" size)
     // 0x1549a966 // -> Segment Info
     // 0x1654ae6b // -> One or more Tracks
 
     // 0x1f43b675 // Cluster
-    if (aData->Length() >= 4 &&
-        (*aData)[0] == 0x1f && (*aData)[1] == 0x43 && (*aData)[2] == 0xb6 &&
+    if (aData->Length() < 4) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+    if ((*aData)[0] == 0x1f && (*aData)[1] == 0x43 && (*aData)[2] == 0xb6 &&
         (*aData)[3] == 0x75) {
       return NS_OK;
     }
     // 0x1c53bb6b // Cues
-    if (aData->Length() >= 4 &&
-        (*aData)[0] == 0x1c && (*aData)[1] == 0x53 && (*aData)[2] == 0xbb &&
+    if ((*aData)[0] == 0x1c && (*aData)[1] == 0x53 && (*aData)[2] == 0xbb &&
         (*aData)[3] == 0x6b) {
       return NS_OK;
     }
-    return NS_ERROR_NOT_AVAILABLE;
+    return MediaResult(NS_ERROR_FAILURE, RESULT_DETAIL("Invalid webm content"));
   }
 
   bool ParseStartAndEndTimestamps(MediaByteBuffer* aData,
                                   int64_t& aStart, int64_t& aEnd) override
   {
     bool initSegment = NS_SUCCEEDED(IsInitSegmentPresent(aData));
 
     if (mLastMapping &&