Unknown type error for interface in contract dependency

[AntonD3]

Version Information

• vyper Version (output of vyper --version): 0.3.3

What's your issue about?

Source codes:

C.vy:

import C1 as C1

@internal
def t1(a: address) -> address:
    return a

@external
def test(_c1: address) -> (address, address):
    c: address = create_forwarder_to(_c1)
    C1(_c1).init_(convert(9, address))
    x: address = C1(c).bla()
    y: address = self.t1(convert(7, address))
    return x, y

@external
def t2() -> address:
    return convert(9, address)

C1.vy

interface C1:
    def bla() -> address: nonpayable
    def init_(x: address): nonpayable

bla: public(C1)

@external
def init_(x: address):
    self.bla = C1(x)

When I try to compile C.vy, I get the next error:

vyper.exceptions.UnknownType: No builtin or user-defined type named 'C1'. 
  contract "C1", line 5:12 
       4
  ---> 5 bla: public(C1)
  -------------------^
       6

[15205] Failed to execute script 'vyper_compile' due to unhandled exception!