Alphanumeric column size cannot be decreased [Message 34-10]

Hi All,

While studing thr e-learning material, under "Customization" in the section "User Define Fields and Table" in the demo of "Creating user define tables"

Firstly they have created a user define fields with Set valid values for field with size 15 as per tutorial,

again when i m unchecking the three check boxes and checking in "Set linked table" the size of Alphanumeric automatically reduce to 8 from 15.

when click on update shows " Alphanumeric column size cannot be decreased [Message 34-10]" Error.

In tutorial the same thing is happening but no error is displayed.

Can any one help me??

Thanks and Regard,

Vivek