Warning (18029): Output pin [...] cannot be tri-stated

Warning (18029): Output pin "_subnet_pin_68" driven by bidirectional pin "sram_bus[6]" cannot be tri-stated


The Quartus help says:

CAUSE:	The specified output pin is driven by both the specified bidirectional pin and tri-stated logic. However, the current device does not support a tri-stated output pin. When the output pin is not driven by either the logic or the bidirectional pin, its output value is undefined.
ACTION:	If the output pin is always driven by either the logic or the bidirectional pin, no action is required. Otherwise, insert an extra tri-state buffer before the output pin.

I have a problem.I doubt that what Output pin "_subnet_pin_68" means. Where is _subnet_pin_68?I can't find it in the design.
Here are the RTL schematic about the bidirectional pin.