Matrix Method for Analysis of Trusses

This is what I currently have and I keep getting an error saying the following.

>> autograder

Index exceeds the number of array elements (1).

Error in aero215_project_kang (line 114)

sigma(i) = (E(i)/length(i)).*dot(change_in_u,unit_vectors(i,:));

Error in autograder (line 64)

[u, F, sigma] = aero215_project_kang(nodetable, connecttable, dof);

so I assume, Error in autograder file comes from the code and I just don't get what it means by index exceeds the number of array elements(1) and where this program is caused.