Unable to define + generically?

Question

I'm following Andrea Ferretti's tutorial and am trying to define + for my own type.

My tuple is:

TUPLE: myBox a ;

If I create my own generic word

GENERIC: my+ ( a b -- c ) 
M: myBox my+ a>> swap a>> swap + myBox boa ;

then I can use it to add to add the contents of two myBoxes together

6 myBox boa 7 myBox boa my+

which outputs

--- Data stack:
T{ myBox f 13 }

However, if try to define + itself for myBox with:

M: myBox + a>> swap a>> swap + myBox boa ;

then, when I try to use it:

6 myBox boa 7 myBox boa +

I get the error:

No suitable arithmetic method
"left" T{ myBox f 6 }
"right" T{ myBox f 7 }
"generic" + 

If I go to the help page for + I even see my definition listed amongst the others (ie bignum, complex, fixnum, float, and ratio).

How can I define the generic word + for my own tuple?