Error code when trying to rig puppet

[AJSiemers]

I try to rig a puppet of a reindeer, but I keep getting the following error code: 

Internal error code: (1109000933:370+8) bad argument#1 to 'Localize'(string expected, got nil)

I can't add any behaviour, the only thing it will do is "motion lines"

What am I missing