"not a valid texture file name" error generated when importing Blender mesh into Unity

This is the error I'm getting:

'.' is not a valid texture file name on asset 'Assets/Models/Planet Original Large.blend' on material 'Mat_Tiled_Grass1__Terrain' of renderer 'Sphere'. The file will be ignored.

The asset had been importing fine as an untextured mesh. I then applied a material using Texture Paint in Blender. I saved the asset into the Assets folder as a Blender file as usual. When the asset had finished updating it generated the error above.

I should add that the mesh looks good and appears correctly textured in Blender.

I assume I've done something foolish when creating my Blender material. Are there any obvious causes of this which I might be tripping over?