No psi interface generated for recursively-only used rule

[imkiva]

Given the following BNF

{
  tokens = [
    ID = 'regexp:[a-zA-Z_][a-zA-Z0-9_]*'
    NUMBER = 'regexp:[0-9]+'
    LAMBDA = 'lambda'
    TO = '->'
  ]
  extends("expr|.+Expr") = expr
}
expr ::= lambdaExpr | literalExpr
lambdaExpr ::= 'lambda' expr '->' expr
literalExpr ::= ID | NUMBER

The expr is only used in its child rules (declared in the extends(...) clause), and after Ctrl + Shift + G, Grammar kit generated the following implementation:

It seems that GK cannot find the psi interface for expr rule. But everything works again after adding a rule that externally uses expr like

useIt ::= expr

I guess this is not the expected behavior, or there should at least be error reporting for this situation.