Help with an RMAN restore

Ok, so I'm trying to restore a datafile from an rman backup.  It's on
oooooold crap!  It's 8.1.7 on SunOS 5.7.  Here is my syntax:

run{
allocate channel ch1 type "SBT_TAPE"
parms 'ENV=(NSR_SERVER=satpcztape1.domain)';
sql 'ALTER DATABASE DATAFILE 7 OFFLINE';
restore datafile 7;
recover datafile 7;
sql 'ALTER DATABASE DATAFILE 7 ONLINE';
release channel ch1;
}

Here is what I'm getting when I run it:

RMAN-03022: compiling command: allocate
RMAN-03023: executing command: allocate
RMAN-08030: allocated channel: ch1
RMAN-08500: channel ch1: sid=16 devtype=SBT_TAPE
RMAN-08526: channel ch1: BMO v3.0
RMAN-03022: compiling command: sql
RMAN-06162: sql statement: ALTER DATABASE DATAFILE 7 OFFLINE
RMAN-03023: executing command: sql
RMAN-03022: compiling command: restore
RMAN-03022: compiling command: IRESTORE
RMAN-03026: error recovery releasing channel resources
RMAN-08031: released channel: ch1
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure during compilation of command
RMAN-03013: command type: restore
RMAN-03002: failure during compilation of command
RMAN-03013: command type: IRESTORE
RMAN-06004: ORACLE error from recovery catalog database: ORA-01460:
unimplemented or unreasonable conversion requested

 I don't get it.  I've been looking up that last error in the stack and it
keeps telling me that I have some kind of conversion (TO_CHAR, etc, etc...)
and I don't!!

Please tell me someone has seen this and knows how to fix it.  I want to get
this done and go to sleep!!

Thanks,

Scott

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