Object reference error in SIFT feature detector

I am currently trying to get the features from the following image using the SIFT feature detector algorithm (sift.q) implemented on quasar. By debugging the process to detect the features of the image, I get the following error on line 119 of the sift_detector function. I tried to execute the portion of code where the error presents for m=0..size(list,0)-1 upstream the debugging point and it executes without problems. I also tried using different images but the results is the same. Any help on this bug will be much appreciated. Thank you!