Why does simple Summation give #VALUE!?

I am getting the error message "#VALUE!" when I attempt a simple summation - either using SUM() or direct cell addition A1+B1+C1+D1 with some blanks in some of the cells (see sample spreadsheet) - but only in some rows

I suspect there's something odd about the cells containing the numbers or blanks, but I cannot see it.

I fear it is something simple, but I've tried for some time to find what's wrong and failed. I would appreciate a fresh pair of eyes to look and hopefully tell me what I'm doing wrong.