treenode ID attribute issue: X "is not a valid identifier"

[n_o_s_p_a__m]

What is a valid identifier that can be assigned to the ID property?

Numbers don't work, letters don't work. I am at a loss here. What is
supposed to go in the ID field?

 

[n_o_s_p_a__m]

Ok looks like the only thing that works is: A1, A2, etc. F'in bizarre.