restore (validate) doesn´t work if i use allocate channel

Hi DBA´s

have problems restoring a database (validate) from tape -backup

try to restore validate a database point in time.
It works only if i don´t use "allocate channel´s" in my rman run - block.
Example (that works):
run {
set until time "to_date('21-01-2013_12:10:00','dd-mm-yyyy_hh24:mi:ss')";
restore database validate;
}

As soon as is use the "allocates" (what we normaly do) rman raises an error
Example (that doesn´t work):
run {
allocate channel c1 device type SBT_TAPE;
allocate channel c2 device type DISK;
set until time "to_date('21-01-2013_12:10:00','dd-mm-yyyy_hh24:mi:ss')";