--- a/content/base/src/nsObjectLoadingContent.cpp
+++ b/content/base/src/nsObjectLoadingContent.cpp
@@ -2268,19 +2268,19 @@ nsObjectLoadingContent::LoadFallback(Fal
 
   //
   // Fixup mFallbackType
   //
   nsCOMPtr<nsIContent> thisContent =
   do_QueryInterface(static_cast<nsIImageLoadingContent*>(this));
   NS_ASSERTION(thisContent, "must be a content");
 
-  if (!thisContent->IsHTML()) {
-    // Don't let custom fallback handlers run outside HTML
-    LOG(("OBJLC [%p]: Non-HTML content, forcing eFallbackAlternate", this));
+  if (!thisContent->IsHTML() || mContentType.IsEmpty()) {
+    // Don't let custom fallback handlers run outside HTML, tags without a
+    // determined type should always just be alternate content
     aType = eFallbackAlternate;
   }
 
   /// XXX(johns): This block is just mimicing legacy behavior, not any spec
   // Check if we have any significant content (excluding param tags) OR a
   // param named 'pluginUrl'
   bool hasAlternateContent = false;
   bool hasPluginUrl = false;