Shield wall formation for spearmen?

[Sir Heavy]

How do I make spearmen use shieldwall like the iron band legionaries use? I tried once but when I start up the game it crashes.

[k/t]

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type Italian Militia
dictionary Italian_Militia ; Italian Communal Militia
category infantry
class spearmen
voice_type Light
banner faction main_spear
banner holy crusade
soldier Spear_Militia, 80, 0, 0.95, 0.30
officer BC_EURO_captain
officer northern_captain_early_royal
mount_effect horse +4, camel +4
attributes sea_faring, hide_forest, can_withdraw
move_speed_mod 0.9
formation 1.2, 1.2, 2.4, 2.4, 5, square, shield_wall
stat_health 1, 1
stat_pri 2, 1, no, 0, 0, melee, melee_blade, piercing, spear, 90, 1
;stat_pri_ex 0, 0, 0
stat_pri_attr light_spear
stat_sec 0, 0, no, 0, 0, no, melee_simple, blunt, none, 25, 1
;stat_sec_ex 0, 0, 0
stat_sec_attr no
stat_pri_armour 2, 1, 6, flesh
;stat_armour_ex 0, 4, 5, 7, 1, 6, 6, flesh
stat_sec_armour 0, 0, flesh
stat_heat 6
stat_ground 1, -1, 3, 0
stat_mental 6, low, trained
stat_charge_dist 10
stat_fire_delay 0
stat_food 60, 300
stat_cost 1, 300, 100, 50, 50, 330, 4, 70
stat_stl 4
armour_ug_levels 1, 2, 3
armour_ug_models Italian_Militia, Italian_Militia, Italian_Militia
ownership milan, pisa, venice, papal_states, slave, sicily
era 0 milan, pisa, venice, papal_states, slave, sicily
era 1 milan, pisa, venice, papal_states, slave, sicily
era 2 milan, pisa, venice, papal_states, slave, sicily
recruit_priority_offset 15
;unit_info 5, 0, 7