Am 07.01.2023 um 19:56 schrieb Sven Schreiber: First of all, in the subject line of course it should be "error message" instead of error matrix. OK , what about something like the following:  if (gretl_is_null_matrix(A)) {         *err = E_INVARG;         return NULL; } else if ( A->rows != A->cols) {         *err = E_NONCONF;       return NULL; } From what I see in lib/src/gretl_errors.c, this means "Matrices not conformable for operation". Or would it make sense to create a new error type, say E_NONSQ or so, saying that the matrix isn't square? thanks sven