'a is unbound

 96  type 'a binop_sig = 'a value -> 'a value -> 'a instr
 97
 98  type ibinop_sig = [< `Int of int | `Vector of int * [`Int of int]] binop_sig

line 98, characters 7-78:
Error: A type variable is unbound in this type declaration.
In type ([< `Int of int | `Vector of int * [ `Int of int ] ] as 'a) binop_sig
the variable 'a is unbound