M4800 Ram upgrade - Computer not starting

Hello,

My m4800 initially came with 8GB of RAM (4GB X 2 Samsung)

I then purchased 2 x Kingston 8GB 1.35v HyperX dimms (both of which I've installed in the primary slots.

When I put the Samsung into the secondary slots, the computer fails to start. Just a complete blank screen with the BIOS log showing "Alert! Memory configured incorrectly"

I've troubleshooted by removing one samsung dimm (from the top slot in the secondary) and the computer starts.

Wondering what's going on.