Trying to monitor port on Cisco 1841

Anybody know what's going on here? I am trying to monitor f0/0 to f0/1, when I go to conf t, I try this: ROUTER(config)#mon session 1 source int f0/0 ^ % Invalid input detected at '^' marker.

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A 'sh int f0/0' sure works fine. Why won't this?

Here I am using the trusty '?', just to make sure I am entering the command right, and it appears that way, except when I enter it, I get that error. Example:

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ROUTER(config)#mon session 1 source int ? FastEthernet FastEthernet IEEE 802.3

ROUTER(config)#mon session 1 source int f ? FastEthernet interface number

ROUTER(config)#mon session 1 source int f 0 ? % Unrecognized command

108BISBEE(config)#mon session 1 source int ? FastEthernet FastEthernet IEEE 802.3

ROUTER(config)#mon session 1 source int f? FastEthernet

ROUTER(config)#mon session 1 source int f0? /

ROUTER(config)#mon session 1 source int f0/? FastEthernet interface number

ROUTER(config)#mon session 1 source int f0/0 ^ % Invalid input detected at '^' marker.

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If anyone has any ideas, I would greatly appreciate it!

Here is part of the sh ver:

--------------------------------------------------------------------- Cisco IOS Software, 1841 Software (C1841-ADVSECURITYK9-M), Version 12.4 (3i), RELEASE SOFTWARE (fc2) Technical Support:

(c) 1986-2007 by Cisco Systems, Inc. Compiled Wed 28-Nov-07 18:48 by stshen

ROM: System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)

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Thanks for any help, ponga