diff --git a/Lib/chunk.py b/Lib/chunk.py
index 0a9abd255a6..320339ef741 100644
--- a/Lib/chunk.py
+++ b/Lib/chunk.py
@@ -61,6 +61,7 @@ def __init__(self, file, align = 1):
             self.chunksize = struct.unpack('>l', file.read(4))[0]
         except struct.error:
             raise EOFError
+        self.chunksize = self.chunksize - 8 # subtract header
         self.size_read = 0
         self.offset = self.file.tell()