The ClientCredentialType configuration cannot be parsed

Dear All

Can anyone please help me to find out the cause for below message prompted when i run RTC Client (NAV 2009 R2).

'The ClientCredentialType configuration cannot be parsed into a valid NavClientCredentialType.


Below from 'CustomSettings.config'.
<!--
The type of client credential used for authentication. (Values are Windows/UserName)
-->
<add key="ClientCredentialType" value="Windows"></add>

Appreciate your help.

Thank you.

Regards
Nasheer.