Material UI Grid item not working properly

I have:

      <div style=>
        <Grid container spacing={0} style=>
          <Grid item xs={12} sm={6}>
            Left Side
          </Grid>
          <Grid item xs={12} sm={6}>
            right Side
          </Grid>
        </Grid>
      </div>

You can see the generated styles are just MuiGrid-item with no reference of the size. What am I doing wrong?

from Material UI Grid item not working properly