How to rotating a rectangle entity without skew?

CesiumJS
1

Hey, I am trying to rotate a rectangle around its center without skewing whenever it goes vertical. The code snippet below demonstrates the skewing. How to avoid it so it stays the same aspect ratio regardless of angle?

        let rotation = 90;

        function getRotationValue() {
            return rotation += 0.01;
        }

        const center = Cartesian3.fromDegrees(-0.1, 55.53645500284618, 0);

        const entity = viewer.entities.add({
            rectangle : {
                coordinates : DataLoader.createRectangle(center, 100, 100),
                material : new Cesium.ImageMaterialProperty({
                    image : '/img.png'
                }),
                rotation : new Cesium.CallbackProperty(getRotationValue, false),
                stRotation : new Cesium.CallbackProperty(getRotationValue, false), // Need this to rotate texture
            }
        });