I'd like to force my exponential fit to start at [0 0]

So, I've got some noisy data. When averaged, the data fit nicely along a double exponential fit, starting off with with a positive slope that decays to near 0 (horizontal line).

However, when I want to fit the same curve to the individual traces, things aren't so neat. Sometimes, the slope is initially negative and shallows out, other times, it's flat and then increases exponentially at the end.

How can I force the exponential fit that I want? It seems like 'Startpoint' is a good starting point (heh).

Here's my code:

[u,gof] = fit(time,y,'exp2');

And this doesn't seem to work:

[u,gof] = fit(time,y,'exp2','Startpoint',[0 0]);

??? Index exceeds matrix dimensions.

Any thoughts would be appreciated.

Thanks, Stefan