type parsing fails for array of interfaces

[mhorsley30896]

Version Information

• vyper: 0.3.1
• OS: linux
• Python: 3.9.7

What's your issue about?

When I try compile the following code

# @version 0.3.1

interface MyInterface:
    def some_func(): nonpayable

my_interface: MyInterface[3]
idx: uint256

@external
def __init__():
	
	self.my_interface[self.idx] = MyInterface(ZERO_ADDRESS)

@external
def use_an_interface():
	
	pass

I get the following error

Error compiling: contracts/MyContract.vy
vyper.exceptions.TypeCheckFailure: Subscript node did not produce LLL. vyper.ast.nodes.Subscript:
     11
---> 12 	self.my_interface[self.idx] = MyInterface(ZERO_ADDRESS)
---------^
     13