寝ていません vs 寝ていない

[Pep95]

English
My older brother is not sleeping now.

Structure

• 今、兄は______

View on Bunpro

Hey, I keep getting this grammar wrong, and I’m wondering what the mistake in my thought process is.

So the verb is continuous, and 寝る is ichigodan, so the form should be 寝ている.
For －いる, the present form can be conjugated into:

This review does not specify whether it should be a polite or plain form, so I’d say that either the いません or いない are applicable here.

Despite this, only 寝ていません is accepted in this exercise, but 寝ていない is not. What am I not seeing here?

EDIT:
In this exercise, the conjugation that I expected is actually used.

[mrnoone]

Hey

You actually found a bug, your way of thinking is correct. There was no alternative answer added.

We have fixed it.

Thank you and sorry for the inconvenience.

[Pep95]

Ah! Then I’m not going mad Thank you for fixing it