Uninitialized Image error message

Hello,
Can anyone help me with this. I am baking images on an object with about 20 different materials. All was working fine but arriving at the 3rd time. I got this error message “Uninitialized image from object plane”.

The only option that is working now is for me to separate the faces I want to bake and reduce the material on this part to one and then bake.

Wish I could do otherwise as you do see some kind of slight difference in the tone of each baked images now when I use the baked textures to the mesh.