SQL & PL/SQL

have the pleasure of learning the benefits of analytical functions and hope to get some help

The case is as follows:
Different projects gets funds from different sources over several years, but not from each source every year.
I want to produce the cumulative sum of funds for each source for each year for each project, but so far I have not been able to do so for years without fund for a particular source.

I have used this syntax:

SUM(fund) OVER(PARTITION BY project, source ORDER BY year ROWS UNBOUNDED PRECEDING)
I have also experimented with different variations of the window clause, but without any luck.

This is the last step in a big job I have been working on for several weeks, so I would be very thankful for any help