diff --git a/src/PIL/PngImagePlugin.py b/src/PIL/PngImagePlugin.py
index 81a9e36af..2c51caf47 100644
--- a/src/PIL/PngImagePlugin.py
+++ b/src/PIL/PngImagePlugin.py
@@ -630,6 +630,7 @@ class PngImageFile(ImageFile.ImageFile):
 
     format = "PNG"
     format_description = "Portable network graphics"
+    __frame = 0
 
     def _open(self):