EIGRP Autonomous-System number must be provided

Sometimes you can face error "EIGRP Autonomous-System number must be provided" during dynamic routing configuration...

(config)#router eigrp 100
(config-router)#address-family ipv4 vrf VRF_1
(config-router-af)#^Z
%Warning: EIGRP Autonomous-System number must be provided

Interesting here that for me (on example on Cisco ME3400) context help inside vrf routing instance didn't show any command that could solve the problem.

Solution is to enter autonomous system number (even if no such command in context help, it will be accepted):

(config-router-af)#autonomous-system 100