I am running a bivariate GREML analysis for 2 binary traits using the latest GCTA 1.25.2. I met the problem that "Error:the X^t * V^-1 * X matrix is not invertible.Please check the covariate(s) and/or the environmental factor(s)". I know that this suggested that the matrix is not positive. I then tried to add an option --reml-bendV, however it seems that this option does not work for the bivariate analysis. May I ask for any possible solution for this?