draggable - grid: [x,y] and containment incompatible

If I use the grid[x,y] property for a jQuery draggable object is not able to contain itself to a containment.

e.g.

.draggable ({
    helper: 'original',
    grid: [50,50],
    containment: 'parent' 
});

does not contain the element to the parent div tag

On the other hand, the following are working:

.draggable ({
helper: 'original',
    //grid: [50,50],
    containment: 'parent'
});

contains it to the div element.