I tried all the possible options in random.method, but I still got the same message - 

    Error in solve.default(vcov(x)[names.coefs_wo_int, names.coefs_wo_int],  : 
    system is computationally singular: reciprocal condition number = 9.29205e-18

The reciprocal condition number changes for the different options. 

However, when I log transformed one of the variables, the issue didn't occur.