Adding a numeric value turns the expression into an int because of type precedence rules. The fact that 'abcd' does not readily convert into an int does not stop this nor generate a run-time error.

So we have a NULL of type int as the first argument and a string which is readily convertible into an int as the second argument and we get an int out of ISNULL.

Try adding one to the result. Or '1'.

Using COALESCE instead of ISNULL also returns an int. The difference is usually in the length of the varchar which is returned and this is sometimes subtle enough to sneak into production code waiting to bite you.