Cannot subtract i from a lsit
I have the list1 defined by this formula: Sequence[b (Mod[n, a] + ί floor(n / a)), n, 0, a² - 1]
And when I type the command list1-i, the result is undefined. On the other hand, list1+2; list1-3; list1+5i; list+i all work fine. Moreover if I define a complex number Z, and a list2=list1+Z, I am able to drag Z to everywhere, list2 stays defined. However list1-Z is always undefined. But the most interesting part is when I define the list3=list1+(-Z). It works, and when I double click on list 3 it says it is defined by list1-Z. list1+(-i) works as well.