Wdt19tbs undersize power adapter on xps17 9710

I turned on my XPS17 9710 powered by a WDT19TBS and original Dell power supply  provided with that docking station. It booted to a bios level error indicating 

a 15 watt power supply was

attached when the WDT19 was providing power. Cycling power to the docking station fixed the problem for now. I’ve seen older references to this problem being caused by firmware issues, 

but nothing that is recent  or related  to the current firmware. 
Any ideas?