Why am I getting the error that the 'dimensions of arrays concatenated are not consistent'?

I have a list of DOY numbers that also contain the hours and minutes that I want to convert to datetime. The variable is DOYfilt and has size 1408x1. On the 434th number in the loop, matlab gives me an error: 'Error using vertcat: Dimensions of arrays being concatenated are not consistent.'. What would be going on in the loop that would cause this when it was working beforehand?

u10_date=[];
for i=1:length(DOYfilt)
    date=datestr(DOYfilt(i) + datenum('2012/01/01'));
    u10_date=[u10_date;date];
end