--- a/dom/media/MediaMIMETypes.cpp
+++ b/dom/media/MediaMIMETypes.cpp
@@ -23,16 +23,19 @@ MakeMediaMIMEType(const nsAString& aType
   nsContentTypeParser parser(aType);
   nsAutoString mime;
   nsresult rv = parser.GetType(mime);
   if (!NS_SUCCEEDED(rv) || mime.IsEmpty()) {
     return Nothing();
   }
 
   NS_ConvertUTF16toUTF8 mime8{mime};
+  if (!IsMediaMIMEType(mime8)) {
+    return Nothing();
+  }
 
   return Some(MediaMIMEType(mime8));
 }
 
 Maybe<MediaMIMEType>
 MakeMediaMIMEType(const nsACString& aType)
 {
   return MakeMediaMIMEType(NS_ConvertUTF8toUTF16(aType));
@@ -98,16 +101,19 @@ MakeMediaExtendedMIMEType(const nsAStrin
   nsContentTypeParser parser(aType);
   nsAutoString mime;
   nsresult rv = parser.GetType(mime);
   if (!NS_SUCCEEDED(rv) || mime.IsEmpty()) {
     return Nothing();
   }
 
   NS_ConvertUTF16toUTF8 mime8{mime};
+  if (!IsMediaMIMEType(mime8)) {
+    return Nothing();
+  }
 
   nsAutoString codecs;
   rv = parser.GetParameter("codecs", codecs);
   bool haveCodecs = NS_SUCCEEDED(rv);
 
   int32_t width = GetParameterAsNumber(parser, "width", -1);
   int32_t height = GetParameterAsNumber(parser, "height", -1);
   int32_t framerate = GetParameterAsNumber(parser, "framerate", -1);