"None[0]" error in series expansion of tensor components

[Alex Cassem]

Hello everyone.

I am doing what seems like it should be straightforward computation by checking whether or not a metric in a solution to the vacuum equations, just R_{ab} = 0. The metric is supposedly a second order gravitational wave solution, to which I am trying to see if R_{ab} = 0 up to cubic order in the amplitude (or expansion parameter rho).

However, the issue I am running into can be seen in the notebook attached and looks like this: 

I am unsure what is causing this issue and would love any input (I was even trying to do it with just a single component of the RicciCD tensor, but I was running into syntax issues). 

Thank you for any input/help.

-Alex Cassem