Unencoded payload works fine, but encoded payload crashes.

[Jordan Trover <shurimaxx () hotmail com>]

I didn't know where to ask for help on this, but I though here would be the best place to ask.
I started learning MASM a few days ago, and I wanted to test executing some payloads from metasploit with masm.
Here's the code: http://pastie.org/private/oyfaky8xttqllgfjlaopzg
You can see there are two payloads there, the first one (commented) is an unencoded payload I generated with:
              ~$ msfpayload windows/exec CMD=calc.exe EXITFUNC=none C
This one works fine, but when I try encoding the payload (the second uncommented payload in the code) with:
              ~$ msfpayload windows/exec CMD=calc.exe EXITFUNC=none R | msfencode -t c
The compiled binary crashes when executing it. It seems to be because of a memory access violation. I loaded it into a 
debugger and the instruction where it crashes is : XOR DWORD PTR DS:[ESI+17],EAX
But ESI is 00000000 at that moment.
I hope someone could help me figure out how I can run the encoded payload just like I do with the normal unencoded 
version.
Thanks.

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