NullReferenceException at OnEntityTakeDamageError

When i do: /jail send "playername" 10 "testing" "prison"
nothing happens and in the console it throws up all kinds of errors like:

(17:18:01) | Failed to call hook 'OnEntityTakeDamage' on plugin 'Jail v4.0.5' (NullReferenceException: Object reference not set to an instance of an object)
at Oxide.Plugins.Jail.OnEntityTakeDamage (BaseCombatEntity entity, HitInfo info) [0x00012] in <43e61bcbfdce4549b0c35613b8085ac7>:0
at Oxide.Plugins.Jail.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00452] in <43e61bcbfdce4549b0c35613b8085ac7>:0
at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <80b90e8213db44b29ec2d4111764172c>:0
at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <ec05e0208c9149bba43236ca58fea105>:0
at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <ec05e0208c9149bba43236ca58fea105>:0
[Error] Failed to call hook 'OnEntityTakeDamage' on plugin 'Jail v4.0.5' (NullReferenceException: Object reference not set to an instance of an object)
at Oxide.Plugins.Jail.OnEntityTakeDamage (BaseCombatEntity entity, HitInfo info) [0x00012] in <13eabc42573743a58f53a0a8441368a4>:0

at Oxide.Plugins.Jail.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00452] in <13eabc42573743a58f53a0a8441368a4>:0

at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <e23ba2c0f246426296d81c842cbda3af>:0

at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <50629aa0e75d4126b345d8d9d64da28d>:0

at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <50629aa0e75d4126b345d8d9d64da28d>:0