What IOS are you running? I'm running:
edge-1811w#sh ver
Cisco IOS Software, C181X Software (C181X-ADVIPSERVICESK9-M), Version 12.4(15)T1, RELEASE SOFTWARE (fc2)
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edge-1811w#conf t
Enter configuration commands, one per line. End with CNTL/Z.
edge-1811w(config)#interface Dot11Radio0
edge-1811w(config-if)#mbssid