Wrong message "You can't send groups under 50 soldiers"

[stratmen]

While trying to send second group of soldiers I get a message  "You can't send groups under 50 soldiers", although there are plenty of them. The group had mixed light and heavy soldiers. When I tried to send just light solders it worked.

Military academy level is 6.

[Starbuck]

you can't send an army in mission if any types of troops comprising it is less than 50

1000 soldiers : 950 light spears and 50 archers .... fine
1000 soldiers : 951 light spears and 49 archers .... not fine