optgroup label name functionality not working

Hi,

I've tried to modify the select box which returned an error. I entered 6 values in my select box. For example Alphabets,A,B,Numbers,1,2. Then after i moved A,B as sub option for alphabets and 1,2 as sub option for Numbers. I mean The values Alphabets and Numbers which i entered should look like optgroup label name. But after doing the above explained and saving the form returns an error. The values i can see in the select box are not the ones which i entered but a text value "Array".

Can someone help me on this.