error combobox showing System.Data.Entity.DynamicProxies.Kel

Question

Hello Everyone. My name is Oktavianus Misro Adrianto, and I am new to programming stuff. 

I have a datagridview contain students infromation from database sql server. now i want to add a combobox to filter the data by class (Students class). I've bind the combobox to database, but each time i select the class, it shows System.Data.Entity.DynamicProxies.Kelas_A3DCF52D2FFF45F41FF2959A996BA8C30B803FE1D8512FD3EA3C74ECE76D3703.

I appreciate if anyone could help me to figure out the problem.