Where() throws IDbAsyncEnumerable not implemented

I have an Repository. For Unitest I use an Fake-Repository that returns a FakeDbSet (Implementation found on internet) with IDbAsyncEnumerable-Implementation.

My Fake-Methode locks like:

public IQueryable<Foo> GetAll()
{
var foo=new Foo();
return new FakeDbSet<Foo>() { foo }.AsQueryable();
}

So this Code works in UnitTest:

Foo[] foo = await MyRepo.GetAll().ToArrayAsync();

but this don't

Foo[] foo = await MyRepo.GetAll().Where(x=>x.Id==1).ToArrayAsync();

Error: The source IQueryable doesn't implement IDbAsyncEnumerable.

Because the Where-Methode returns IQueryable.

What can I do?