IfThenElse inside Select using context not working as intended #1038

Hello, I looked for a bit but was unable to find anything related to this.
I have this code snippet :

from construct import *

b1 = IfThenElse(lambda ctx: ctx._params.get('ctx') == 1, Int8ul, Int16ul)
a = Struct(
    "1" / Int8ul,
    "2" / Select(b1, Bit)
)
context = {"ctx": 1}

b = a.build({"1": 1, "2": 2}, **context)
print(len(b))  # Output: 3

I would expect to have the length of b to be 2 as my ctx provided is 1 but not it goes to the else part of the IfThenElse statement.
The behavior is correct when I remove the Select.