Shading the ArcGIS Map Leads to Triangles Forming

Hi there,

I am having a problem using the ArcGIS map chart in Power BI. The postcode sector displays with the correct shape, until I drag something into the colour bit. Adding a color variables causes all shapes, that were previously in their correct postcode-sector shape, to turn into different sized right-angled triangles.

The steps taken to get to this:

1. Load in the postcode data
2. Create an ArcGIS map
3. Drag the postcode sector variable into the location section of the map
4. It fails to locate them, so we go into 'more options >> edit >> location type' and set it to 'represent location as: boundaries' and set location as 'one country' 'united kindom', location type as 'postcode sectors'
5. Once this is filled in I then drag a variable into the 'color' section and the previous postal code shapes all turn into triangles

What needs to be done to stop the triangles forming and the correct, coloured postal code areas are left instead? Has anyone encountered this problem before?

Attached are three images: 

1. What the map looks like before the color variable is added (at postal code level)
2. What the map looks like with the color variable added (triangles present)
3. A screenshot of the data behind the map

If you have any suggestions, let me know. Many thanks.

QS