Set: in part assignment is not a symbol

This is probably a basic question but I am new to Mathematica, so please help me out here.

I have the following piece of code:

lmax = 2; arrlen = lmax + 1; xarr[x_] = Array[x^# &, arrlen, 0]; parr[p_] = Array[p^# &, arrlen, 0]; arr[x_, p_] = Join[xarr[x], parr[p]]; mboot[x_, p_] := Outer[NonCommutativeMultiply, arr[x, p], arr[x, p]] 

Now, I want to set a few particular elements of mboot[x,p] to 0. But when I try, for example:


I get an error saying Set: mboot[x,p] in the part assignment is not a symbol.

It would be great if someone could help me figure out what is going wrong and how I can fix it.