Failed to parse generated code when optimize=true

[farleylai]

I am working on implementing SSIM in Torch7 with autograd.
Everything works as expected.
However, when optimize is set to true, the following error is thrown:

[string "return function(locals, rlocals, vlocals, obj..."]:15: ')' expected near ','
/home/ml/farleylai/Downloads/pkg/torch/install/bin/luajit: ...re/lua/5.1/autograd/runtime/codegen/backend/lua/init.lua:775: failed to parse generated code.
stack traceback:
	[C]: in function 'error'
	...re/lua/5.1/autograd/runtime/codegen/backend/lua/init.lua:775: in function 'generateFn'
	.../install/share/lua/5.1/autograd/runtime/codegen/init.lua:140: in function 'fn'
	.../pkg/torch/install/share/lua/5.1/autograd/optim/init.lua:11: in function 'optim'
	demo-simple.lua:53: in main chunk
	[C]: in function 'dofile'
	.../pkg/torch/install/lib/luarocks/rocks/trepl/scm-1/bin/th:150: in main chunk
	[C]: at 0x00405d50

The sources are attached FYI:

• SSIM loss: ssim.lua.txt
• Training loop: demo-simple.lua.txt

Any ideas?