Cant do “ls” in ftp thru ISA Server

i have ISA server as a proxy.
when i do ftp either from a client behind the proxy, or from the proxy itself to an external ftp server, i could connect but i cant list directories (ls) it returns with error ” ftp: bind :Can’t assign requested address “.
can anyone tell me whta the problem is, i think its a restriction either from the isa or the cisco 2600 router that goes to our isp. i think its prot 20 that needs to be opened up, but how do i do that on the isa?