diff --git a/content/media/MediaResource.cpp b/content/media/MediaResource.cpp
index 8a4b8779319f..c860c5f40720 100644
--- a/content/media/MediaResource.cpp
+++ b/content/media/MediaResource.cpp
@@ -1031,7 +1031,7 @@ private:
 void FileMediaResource::EnsureLengthInitialized()
 {
   mLock.AssertCurrentThreadOwns();
-  if (mSizeInitialized) {
+  if (mSizeInitialized || !mInput) {
     return;
   }
   mSizeInitialized = true;