cannot import a blender rigged model to Unity

I exported a fully rigged character done in Blender. When I try to create a humanoid avatar for it in Unity, however, the engine complains about not finding a head bone.
Has anyone else experienced this? How do I go about fixing it?

can’t see your image. Do you have a head bone? Mecanim requires a specific minimum set of bones in order for re-targeting to work, so you need to design your model to include them.