error when download program in nios

when I download program in nios,the console display the following information

assertion "m_state == STATE_DEBUG" failed: file "nios2oci.cpp", line 157

Using cable "USB-Blaster [USB-0]", device 1, instance 0x00

Pausing target processor: not responding.

Resetting and trying again: D:\altera\81\nios2eds\bin\nios2-download: line 594:

6720 Hangup nios2-gdb-server --instance 0 --sidp 0x02021028 --

id 2039572636u --timestamp 1241272108u --tcpport none --write-pid ./Debug/nios2-

download.pid ./Debug/GigaCard.elf.srec

does anyone know what caused this?Thank you!