j starts at 1. j-1 starts at 0. 0 multiplied by anything is 0. Therefore your first index into v would be 0, which is not a permitted index.
If you add one to the indices to avoid this problem, such as starting with j = 2, then j-1:j+N0-2 would be 1:2+4-2 ==> 1:4 which is plausible. Then you multiply that by dt which is 1/fs so dt = 1/200. This would give you an index of (1:4)/200 which would not be integer indices.
Therefore your problem is not just an "off-by-one" problem: you are confusing indexing (relative positions into an array) with the value that the location is intended to denote. You cannot index at 1/fs: you can only index at a value that is associated with 1/fs .
In most cases, your first index into an array should work out to be either 1 or 2 (unless you are reading from a static data array based upon user input or device readings or date)