Vue3 / typescript: how to type component props for 2d array

I am converting an existing vue project (written in JS) to Typescript. I am mostly getting on well, but am having a problem with passing an array as props to a component. I am using Composition API / SFC.

For one component (which doesn't need an array), I have this structure:

<script setup lang="ts">
  import type { TrackTile } from '../../env.d';

  interface Props {
    tile: TrackTile,
    item: number
  }
  const props = defineProps<Props>()
  console.log('keyItem script setup');
  console.log(props.tile);

</script>

This transpiles without errors and then runs fine, so I assume my basic syntax is OK here. But another component mainGrid needs to be given an array of TrackCell types:

<script setup lang="ts">

  import type { TrackCell } from '../../env.d';
  interface Props {
    grid: TrackCell[][];
  }
  const props = defineProps<Props>()

  console.log('mainGrid script setup');
  console.log(props.grid[0][0]);
    
</script>

The component is called from its parent like this:

 <mainGrid :grid = trackGrid>
 </mainGrid>

In this case I get an error that props.grid[0] is undefined.

As a check to ensure that the trackGrid array (and its constituent trackCells) was defined and populated correctly, I temporarly moved the template for the mainGrid component into its parent, and everything then works as expected. So it appears that it's just the `defineProps' section which is incorrect.

trackGrid is reactive:

export const trackGrid: TrackCell[][] = reactive([] );