Proc#arity strangeness

I was fooling around with some examples using blocks this morning before work, and I noticed that the ruby docs for Proc#arity say:

Proc.new {}.arity          #=>  0
Proc.new {||}.arity        #=>  0
Proc.new {|a|}.arity       #=>  1
Proc.new {|a,b|}.arity     #=>  2
Proc.new {|a,b,c|}.arity   #=>  3
Proc.new {|*a|}.arity      #=> -1
Proc.new {|a,*b|}.arity    #=> -2

But the ruby interpreter I’m using (MRI 1.8.6) actually returns these results:

Proc.new {}.arity          #=>  -1
Proc.new {||}.arity        #=>  0
Proc.new {|a|}.arity       #=>  1
Proc.new {|a,b|}.arity     #=>  2
Proc.new {|a,b,c|}.arity   #=>  3
Proc.new {|*a|}.arity      #=> -1
Proc.new {|a,*b|}.arity    #=> -2

Anyone know why the empty block returns -1 for its arity? Is it a known bug, or is it supposed to work like that and the documentation is wrong? In this case I think the documentation makes a lot more sense.

*** This appears to have been fixed in MRI 1.9 ***