diff -r b898402e75ef Lib/gzip.py
--- a/Lib/gzip.py	Mon May 19 07:34:08 2014 +0100
+++ b/Lib/gzip.py	Fri May 23 13:43:24 2014 +0200
@@ -339,9 +339,9 @@
             data = data.tobytes()
 
         if len(data) > 0:
+            self.fileobj.write( self.compress.compress(data) )
             self.size = self.size + len(data)
             self.crc = zlib.crc32(data, self.crc) & 0xffffffff
-            self.fileobj.write( self.compress.compress(data) )
             self.offset += len(data)
 
         return len(data)