diff --git a/lib/src/room.dart b/lib/src/room.dart
index 0a7cc655..a87f88ef 100644
--- a/lib/src/room.dart
+++ b/lib/src/room.dart
@@ -578,7 +578,7 @@ class Room {
               'thumbnail_info': {
                 'h': thumbnailImage.height,
                 'mimetype': mimeType,
-                'size': thumbnailImage.length,
+                'size': thumbnail.size,
                 'w': thumbnailImage.width,
               }
           }