Cisco Switching/Routing :: Port Monitoring On A 2901 For Purpose Of Packet Capture?

I have always done my port monitoring (SPAN) on Cisco layer 3 switches with no issues. This time I am trying to do this on a Cisco 2901 router:

Cisco IOS Software, C2900 Software (C2900-UNIVERSALK9-M), Version 15.1(4)M2, RELEASE SOFTWARE (fc1)
System image file is "flash0:c2900-universalk9-mz.SPA.151-4.M2.bin
 
I need to have the source port gig0/0 and destination port gig0/1. There is something about the gig port enumeration (slot/port#) that makes the command rejected. It is self explanatory:
 
#sh ip int brie
Interface                  IP-Address      OK? Method Status                Protocol
Embedded-Service-Engine0/0 unassigned      YES NVRAM  administratively down down   
GigabitEthernet0/0         xxx.xxx.xxx.xxx      YES NVRAM  up                    up     
GigabitEthernet0/1         unassigned      YES NVRAM  up                    up     
Serial0/0/0:0              unassigned      YES unset  up                    up     

[code]....
 
It doesn't matter what slot or port number I use, it is always rejected. The command is rejected for Both destination and source gig interfaces. I tried a wide variety of slot/port numbers. To my best understanding the complete port names are: GigabitEthernet0/0 and GigabitEthernet0/1, so why does it think there has to be another digit after 0/0 or 0/1? Does it have anything to do with the Embedded-Service-Engine0/0 being administratively down?