VFadeIn TypeError: len() of unsize object

class Test(InteractiveScene):
    def construct(self):
        # Todo
        sq = Square(side_length=0.5+MED_LARGE_BUFF)
        sq.set_fill(BLUE_D, 0.7)
        sq.set_stroke(BLUE_D, 5)
        self.play(
            DrawBorderThenFill(sq)
        )
        self.wait()
        self.play(
            sq.animate.shift(2*UP+2*RIGHT)
        )

        #new sq
        sqs = VGroup(*(Square(side_length=0.5+MED_LARGE_BUFF) for n in range(3)))
        sqs.arrange(RIGHT)
        sqs.match_style(sq)
        self.play(
            LaggedStart(DrawBorderThenFill(sqs), lag_ratio=0.5)
        )
        self.play(LaggedStart(
            sqs[1].animate.next_to(sq,RIGHT,buff=0),
            sqs[0].animate.next_to(sq,UP,buff=0),
            sqs[2].animate.next_to(sq,UR,buff=0),
        ),lag_ratio=0.4)

        # VFadeIn
        self.play(
            FadeIn(DecimalNumber()),
            run_time=2
        )
        # Why This Error? In VFadeIn??