Discrim function for Linear discriminant analysis problem

Hello, I am using the function "lda" from the discrim toolbox in matlab. However, I'm having the following error :

> In lda.lda at 191 In testLDA at 10 ??? Error using ==> inv Matrix must be square.

Error in ==> lda.lda at 196 S = S*inv(triu(qr(s*v')));

Error in ==> testLDA at 10 F = LDA(X, Y)

My matrix X : p x n where p is the number of nodes and n the number of features. My matrix Y : p x 1 (contains values between 1 and 6 because I have 6 classes).

Does anyone know how to fix this problem ?