Unable to add a new value to one of the segment of "Accounting Flexfield"

Hi,

When I try to add a new value to the value set of one of the segment of "Accounting Flexfield", I get the following error:

"APP-FND-00734: Please enter a valid value for the Account Type segment qualifier using the Segment Qualifiers pop-up window. Or, contact your system administrator to define a default value for this segment qualifier using the Register Key Flexfield form."

What could be the reason for this error ?

Thanks,

Krishna