Type pattern expander in custom syntax class has confusing error

[iitalics]

For some reason, using the generated ~ pattern expanders for types does not work in (define-syntax-class ...). As a simple example,

#lang turnstile

(define-type-constructor List #:arity = 1)

(begin-for-syntax
  (define-syntax-class list-τ
    (pattern (~List _))))

produces the error

pattern: required attribute is not defined: g86
;   in: (pattern (~List _))

I think this has something to do with the temporary generated for use in producing the "Expected _, got: _" error.