Singular matrix warning
In general, a 'singular matrix' error means you either have 

a) A loop of zero impedance elements (for instance inductors, voltage sources)

b) An infinite impedance node (for instance a series connection of two capacitors)

In both cases, judicious use of a small resistor to put some impedance in the loop, which is probably realistic anyway, or a big resistor from the node to ground (10meg, 100meg) will make the simulator maths happy without changing the operation of your circuit too much.