ADG409

I have an ADG409 selecting taps on a resistor divider string in the feedback path of an OpAmp, It is a +-12 volt circuit. And I am occasionally burning holes through the top of the chip package.?.  The top of the string is 3.5K before the top switch and the lowest switch has 539 ohms to ground so I am confident the switch itself is not the cause of the destruction.

My question is, how does the ADG409 handle loss of the Vss connection. 

Vss is normally connected to -12 volt in this circuit. I can conjecture a possible loss of the negative rail because of the pin placements on the boards backplane connector, but am unsure if this would harm the chip.?. Note also that the en (enable) pin is directly connected to +12 volts. If the loss of the -12 volt connection can cause this, would a limiting resistor on the enable line prevent damage? Would placing a reverse biased diode between Vss and ground (locking Vss to ~.3 to .6 volts above ground) alleviate the risk.  I appreciate your input. Thanks.