Overzealous error in phrase/3

[UWN]

?- As = [], phrase(As,Bs).
   As = [], Bs = [].
?- As = [a], phrase(As,Bs).
   As = "a", Bs = "a".
?- As = [], phrase([a|As],Bs).
   error(instantiation_error,must_be/2), unexpected.
?- As = [], call(phrase([a|As],Bs)).
   error(instantiation_error,must_be/2), unexpected.
?- As = [], phrase([a|As],Bs)=G_0,G_0.
   As = [], Bs = "a", G_0 = phrase("a","a").
?- (As = 1, phrase([a|As],Bs))=G_0,G_0.
   error(instantiation_error,must_be/2), unexpected.

[UWN]

?- false,phrase(call(1),Xs).
   error(type_error(atom,1),(=..)/2), unexpected.

[UWN]

Thanks for the reminder, @flexoron Added as #33

[UWN]

?- L=[],(true;phrase([1|L],_)).
   L = []
;     error(instantiation_error,must_be/2), unexpected.
?- call((L=[],(true;phrase([1|L],_)))).
   L = []
;     error(instantiation_error,must_be/2), unexpected.

So this error cannot be from some term-to-body conversion prior to executing any of the goals.