Variant not working in VC

• I have 3 variants of an object.

• One of them Var 1 is version control.
• I want to explore variants in Version control. 
• When I drag and drop var2 on var1 I am asked if I want it as variant of version, I hit variant. 

• If I hit variant I get unknown error 

Edit : It seems like the error can be solved if I manually add a preview to my variant. 

But Absence of preview should not trigger an error, and also as I am drag and dropping an object that has a preview, why is the preview field not already loaded properly ?