Negative variance - using regression. HELP!

I'm currently writing up a scientific project and I've come to a complete standstill when calculating the variance. The formula I am using to calculate Variance (from the textbook) is one that is obtained from Linear regression theory as the variance of the slope of the regression... I have attached it to this post because it isn't a standard formula - it has been adapted for the biological methodology I am using.

Just for reference, to make sense of my formula, I conducted a mark recapture study on individual animals, whereby:
Ct = total number of individuals caught in sample t
Rt = Number of individuals already marked when caught in sample t
Ut= Number of individuals marked for the first time and released in sample t
Mt = Number of marked individuals in the population just before sample t is taken.

I have to do the calculations for 3 different data sets. First two - no problem. Third one.. I am getting a negative value. I've gone over and over and over double checking that I haven't mis-calculated anything but I definitely haven't. What's going on?!! I am SO stuck!