NA's in segmented

Hi,

I?m contacting you because I have some trouble with the slope() function of segmented. When I plot the predicted value everything seems to have worked well. But when I use slope(), the slope for the first segment is a line with 0 and Nas? Also I can get a negative slope whereas on the graph it?s clearly a positive slope?

Here are the lines I?m using :

Mod = lm(as.numeric(data[,"meanMSD25"])~(as.numeric(data[,"Slice"])), weights=1*sqrt(as.numeric(data[,"Detec"])))
  x = (as.numeric(data[,"Slice"]))
  o <- segmented(Mod, seg.Z=~x, psi=NA, control=seg.control(display=FALSE, K=2))

Thanks in advance for your help,

Best regards,

M?lina COINTE