Change 0 result from SUMIFS to something that can be summed with duration format cells

I have a table of data generated by the time monitoring app 'TimeCop'. I'm trying to use Numbers to sumarise this to a series of weekly charts and graphs. Thanks to answers in this forum I'm about half way there, but I've hit a problem I can't find an answer to…

The source data table contains, amongst other things, a column of projects I've been working on, the days I've been working on them, and the duration of each session I was working. My target table has columns for days of the week and rows of projects and at their intersection I've figured out how to use SUMIFS to extract fromt he source table the sum of hours and minutes for each project for a particular day. So far so good.

The problem is when I want to SUM either the rows to get a total time for either a particular day (column) or project (row) I get an error that reads "Values being summed by SUM must be either all durations or all numbers, unless one value is a date." - a bit of poking around seems to suggest that the problem is being caused by cells where the conditions of the SUMIFS result in no match and a 0 is displayed. Despite both the source and target cells being formatted as a duration it seems this is ignored by the SUM. Is there a way to either get the SUMIFS formula to return something other than 0 (ie; 00:00) when no match is found, or to format the SUM to ignore them, or something else…?

I'm an infrequent and inexperienced user of spreadsheets in general and Numbers in particular so it's entirely likely I'm missing something obvious…! Any help much appreciated.