SRAM pin filter warning!

hi,when i fiter a sram pin,some warnings come out,and such as below:

Warning: Can't pack node sram_addr[0] to I/O pin

Warning: Can't pack node sram_addr[0] -- no packable connection between output pin and register

why ,please help me!

i use nios2, quartus4.2, cy12 board made by myself.

thank you!