Undamped natural frequencies

Hello everybody,

I have a question about the undamped natural frequencies for a three degree of freedom system.

The equation of motion for the undamped system is :

M$$\ddot{x}$$+K$$\dot{x}$$=0

The undamped natural frequencies I found is a complex number : $$\omega$$2=-187.29

Is this kind of result is possible? I'm surprised by the result I found since every time I've done a similar exercise before, the frequencies were always real numbers.